Russia's military forces began an attack on Ukraine early Thursday morning, moments after Russian President Vladimir Putin appeared on video to announce a "special military operation," in a bloody conflict now stretching across the embattled eastern region of Donbas and beyond.

Early on, explosions could be heard across parts of Ukraine, including the capital of Kyiv, hours after Ukrainian President Volodymyr Zelensky said he had reached out to Putin seeking peace only to be met with "silence." The volleys included cruise and ballistic missiles, according to Ukrainian officials, who also told reporters that Russian troops landed in the port city of Odessa, which is across the Black Sea from Crimea, and crossed the border into Kharkiv in the northeast, NBC News reported. Ukraine's border guard agency said attacks were also coming from northern neighbor Belarus, where Russia had moved forces "with the use of artillery, heavy equipment, and small arms."

Hours into the assault, Ukraine's military claimed that five Russian aircraft and a helicopter had been shot down, according to CNN, while Russia's state-run news agency TASS denied the claims. Russia's Defense Ministry insisted that only military targets, not civilians and cities, were being attacked and claimed that Ukrainian border guards were not putting up any resistance against Russian troops.

"The Ukrainian border guards are not putting up any resistance to Russian units. The Ukrainian air defenses are suppressed. The military infrastructure of Ukrainian Air Force bases has been degraded," the ministry said. As reported by the Wall Street Journal, a senior Ukrainian Interior Ministry official said, “You can be certain that hundreds of Ukrainian service members have already died from the Russian strikes."

Ukraine's Ministry of Defense shared a statement on social media claiming that the situation is under control, asserting that Russian vehicles were being damaged or destroyed and invading forces were "suffering losses." In an early casualty report, at least eight people were killed and nine others were wounded by Russian shelling, Ukraine's minister of internal affairs said, according to Reuters.

Meanwhile, in the financial sector, stocks took a dive, while oil prices jumped.

President Joe Biden, who approved hundreds of millions of dollars in military assistance to Ukraine in the weeks prior to the attack, is receiving regular updates on the situation in Ukraine, according to the White House. Biden also issued a statement vowing that the "world will hold Russia accountable." With the attack stretching from the predawn hours into daylight and air raid sirens blaring in Kyiv, Zelensky announced that he had just spoken with Biden and declared martial law while urging the Ukrainian people to remain calm.

“Dear Ukrainian citizens, this morning, President Putin announced a special military operation in Donbas. Russia conducted strikes on our military infrastructure and our border guards. There were blasts heard in many cities of Ukraine. We’re introducing martial law on the whole territory of our country. A minute ago, I had a conversation with President Biden. The U.S. have already started uniting international support. Today, each of you should keep calm. Stay at home if you can. We are working. The army is working. The whole sector of defense and security is working," Zelenksy said in a brief video address, according to a CNN translation.

“No panic. We are strong. We are ready for everything. We will win over everybody because we are Ukraine," he added.

Biden said in his own statement that Zelensky reached out to him.

"I condemned this unprovoked and unjustified attack by Russian military forces. I briefed him on the steps we are taking to rally international condemnation, including tonight at the United Nations Security Council," Biden said of his conversation. "He asked me to call on the leaders of the world to speak out clearly against President Putin’s flagrant aggression and to stand with the people of Ukraine. Tomorrow, I will be meeting with the leaders of the G-7, and the United States and our Allies and partners will be imposing severe sanctions on Russia. We will continue to provide support and assistance to Ukraine and the Ukrainian people."

The offensive is the latest escalation in a conflict spanning years, encompassing Russia's invasion and annexation of Crimea eight years ago and, since, Ukraine's forces fighting pro-Kremlin military separatists in eastern Ukraine. In the lead-up to the attack Thursday morning local time, Russia had amassed as many as 190,000 troops along the Ukrainian border, including in Belarus, and Ukraine had reported being subject to cyberattacks.

Ukraine is not a member of NATO, but it does share a border with four NATO member states, and with the attack Thursday, European countries are calling for measures to be taken in the face of a rising threat to the alliance's territory under Article 4.

In his televised address, which aired early Thursday local time soon after Russia issued notices for air travel to avoid northeastern Ukraine, Putin said the new military attack was aimed at the “demilitarization and denazification” of Ukraine, not its occupation.

“I’ve made the decision to conduct a special military operation," Putin said in the address, according to a translation. “Our analysis has concluded that our confrontation with these [Ukrainian] forces is inevitable.”

The Russian leader also warned Western countries against sending any forces to assist Ukraine.

“A couple of words for those who would be tempted to intervene: Russia will respond immediately, and you will have consequences that you never have had before in your history,” he cautioned.

In addition, Putin encouraged Ukrainian forces to lay down their arms and return home.

Dmytro Kuleba, the minister of foreign affairs of Ukraine, said in a tweet: "Putin has just launched a full-scale invasion of Ukraine. Peaceful Ukrainian cities are under strikes. This is a war of aggression. Ukraine will defend itself and will win. The world can and must stop Putin. The time to act is now."

Zelensky delivered an address earlier Wednesday, spoken mostly in Russian, in which he warned that the Kremlin had nearly 200,000 soldiers and thousands of armored vehicles on the border ready to attack his country. He also pleaded with the Russian people, insisting that the two nations did not have to be enemies and arguing that Ukraine is not a threat to Russia. His remarks contrasted the long speech Putin delivered Monday discussing the history of Ukraine and claiming that Ukraine was essentially a colony of Western powers.

"Listen to the voice of reason. The people of Ukraine want peace. The authorities in Ukraine want peace. They want it and are doing everything they can for it," Zelensky said, according to a translation. "But if we are attacked — if someone attempts to take away our land, our freedom, our lives, the lives of our children — we will defend ourselves. We won't attack but defend ourselves. By attacking, you will see our faces — not our backs but our faces."

Biden issued a statement within an hour of Putin's address airing.

"The prayers of the entire world are with the people of Ukraine tonight as they suffer an unprovoked and unjustified attack by Russian military forces. President Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering. Russia alone is responsible for the death and destruction this attack will bring, and the United States and its Allies and partners will respond in a united and decisive way," Biden said.

"The world will hold Russia accountable," Biden added. "I will be monitoring the situation from the White House this evening and will continue to get regular updates from my national security team. Tomorrow, I will meet with my G7 counterparts in the morning and then speak to the American people to announce the further consequences the United States and our Allies and partners will impose on Russia for this needless act of aggression against Ukraine and global peace and security. We will also coordinate with our NATO Allies to ensure a strong, united response that deters any aggression against the Alliance. Tonight, Jill and I are praying for the brave and proud people of Ukraine."

Hours before Putin announced the military operation, a senior defense official told reporters that Russia had “significant offensive missile capability to offensive ground power. ... He has more than two dozen warships in the Black Sea. The majority of them are surface combatants. He has a cruise missile capability. He's got ballistic missile capability arrayed. He’s got armor, artillery, certainly infantries. He’s got special forces.”

TWO FOREIGN MINISTERS FROM NATO ALLIES IN KYIV AS RUSSIA ATTACKS UKRAINE

Marco Rubio, vice chairman of the Senate Intelligence Committee, described in a Twitter thread what he expected Russia to do in its offensive and provided some updates.

"In the hours to come," the Florida Republican said, Russia will "conduct strikes on air defense systems, move to cut off #Kyiv from eastern #Ukraine," and "move to cut off Ukraine’s military forces on the line of contact in the east to prevent them from falling back to defend Kyiv."

Rubio added in a later tweet, "Russian aircraft are now en route to conduct additional targeted strikes on a number of key defense sites through all of #Ukraine."

"#Russia’s airborne forces are attempting to take control of the airport in #Kyiv to they can fly in forces to occupy the capitol city," Rubio also said, adding that "an amphibious assault on the key port city of #Mariupol is now underway" and "ground forces now moving in from Belarus, Crimea & from #Russia."

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Pentagon spokesman John Kirby told reporters Wednesday that Russia is "ready to go" and that its forces could "attack at any time."

Tensions between Ukraine and Russia escalated rapidly in recent days after Putin decided to recognize two occupied Ukrainian territories as independent earlier this week. He also ordered "peacekeeping" troops to go into those areas at the time.

The Ukrainian government previously issued a state of emergency meant to stay in place for at least 30 days.

Biden reimposed the sanctions Wednesday as a punitive measure against Russia for authorizing troops to enter separatist-backed regions of Donetsk and Luhansk, also known as Donbas, in eastern Ukraine. Germany also suspended the certification of the pipeline in response to Russia's hostile actions. Suspension of the pipeline comes at a time when Germany and other European nations are experiencing an energy crisis. An estimated 40% of Europe's energy comes from Russian natural gas.

The sanctions against the pipeline coincided with other sanctions that the United States and Europe imposed on Russia in response to Putin's actions. This includes sanctions that target Putin's inner circle.

Ambassador Bathsheba Nell Crocker, the U.S. representative to the European office of the United Nations, recently warned that the Russians had created a list of Ukrainians to kill or imprison following an invasion.

“We have been warning individuals and groups who we think could be targeted based on our understanding and our knowledge of Russian plans," a senior administration official told reporters earlier this week but did not specify who received such alarms.

U.S. officials reportedly suggested that Zelensky leave Kyiv if an invasion began, according to NBC News.

Secretary of State Antony Blinken, who days ago ordered State Department officials working in Ukraine to withdraw across the border into Poland each night over the threat of an invasion, told NBC News on Wednesday, before the attacks began, that there is "still an opportunity to avert a major aggression, although again, they're in their final stages of being able to engage in one.”

An emergency meeting of the U.N. Security Council took place as the Russian offensive got underway.

Secretary-General Antonio Guterres admitted that he had been wrong to believe "nothing serious would happen" and called on Putin to "stop [his] troops from attacking Ukraine."

The rotating president of the Security Council for February happens to be Russia.

Ukraine’s ambassador to the U.N., Sergiy Kyslytsya, called on the world to stop Russia’s war against his country. The Russian ambassador to the U.N., Vasily Nebenzya, responded: “This isn’t called a war. This is called a special military operation in the Donbas.”

The Security Council meeting concluded with the Ukrainian representative demanding that the Russian representative relinquish his role as chairman, saying, “There is no purgatory for war criminals. They go straight to hell, ambassador.” In response, the Russian said, "We aren’t being aggressive against the Ukrainian people but against the junta that is in power in Kyiv,” and ended the meeting.

NATO Secretary-General Jens Stoltenberg also issued a statement after the Russian attack began.

"I strongly condemn Russia’s reckless and unprovoked attack on Ukraine, which puts at risk countless civilian lives. Once again, despite our repeated warnings and tireless efforts to engage in diplomacy, Russia has chosen the path of aggression against a sovereign and independent country," Stoltenberg said.

"This is a grave breach of international law, and a serious threat to Euro-Atlantic security," he added. "I call on Russia to cease its military action immediately and respect Ukraine’s sovereignty and territorial integrity. NATO Allies will meet to address the consequences of Russia’s aggressive actions. We stand with the people of Ukraine at this terrible time. NATO will do all it takes to protect and defend all Allies."