BEIRUT (AP) -- The unsteady, hand-held video shows several bloodied prisoners, one in boxer shorts, being led into a noisy outdoor crowd and placed against a wall. The prisoners crouch and seem to avert their eyes as men carrying assault rifles shout slogans and take aim. The gunfire lasts for more than 30 seconds.

The international community has accused Syrian President Bashar Assad's forces of war crimes, but the gunmen in this gruesome video were rebels. Their slogans: "Free Syrian Army Forever!" and "God is Great!"

The video, which surfaced online this week, is fueling concerns that opposition fighters are capable of brutality that matches that of the regime they are seeking to topple -- a charge that could badly damage the rebellion's ability to claim the moral high ground in the Syrian civil war.

As rebels gain more territory and a multitude of militias, jihadists and criminals join the fight against Assad, reports of serious human rights abuses committed by armed opposition elements are on the rise.

"As the Free Syrian Army and armed opposition gain more ground control, they are at a crossroad," said Nadim Houry, researcher at the New York based Human Rights Watch.

"They can either go down the route of revenge and killings and replicate the behavior that we have seen by pro-government forces, or take a genuine decision showing that what they're fighting for is not just about revenge but about human rights and justice," he said.

The video sparked international condemnation, including a rare rebuke of rebel tactics from the Obama administration on Thursday.

"This is abhorrent and inconsistent with the type of struggle for freedom and a new Syria that the broad opposition is looking for," State Department spokesman Patrick Ventrell told reporters.

Ventrell said the U.S. was encouraged that opposition commanders have condemned abuses, but stressed that "summary executions committed by any party are abhorrent and inconsistent with international law, and those responsible must be held to account."

Separately, Ventrell also criticized what he described as another "massacre" by Assad's forces, this time in the Damascus suburb of Yalda. U.S. officials, speaking on condition of anonymity because they were not authorized to speak publicly, said civilians were killed execution-style with gunshots to their heads or necks. They said the killings took place in the victims' homes, basements and gardens.

"It is the Assad regime and Assad's forces that have perpetrated the overwhelming amount of violence in Syria, that are responsible for the overwhelming number of civilian casualties," White House spokesman Jay Carney said.

Assad's regime stands accused of a number of massacres in which hundreds of civilians, including women and children, were killed. The Syrian government blames gunmen driven by a foreign agenda for the killings, but the U.N. and other witnesses have confirmed that at least some were carried out by pro-regime vigilante groups, known as shabiha.

Nevertheless, summary executions committed by rebel forces -- albeit on a far smaller scale than the regime's alleged atrocities -- put the West in a difficult position as it seeks to persuade Russia and China to stop blocking tough U.N. action against Assad.

"In the areas they control the rebels bear responsibility for preventing acts of revenge and violence against defenseless persons," German Foreign Minister Guido Westerwelle said Wednesday. "We clearly expect them to be aware of this responsibility."

Representatives of the Free Syrian Army, the Turkey-based umbrella group of Syrian rebels, acknowledged that the executions shown in the video were wrong but said the rebels, unlike Assad's forces, do not attack ordinary civilians.

"The Free Syrian Army does not attack civilians," said spokesman Ahmed Kassaem. "We do not support killings and are not Sadists like the regime."

Activists said those shown killed in the video were not ordinary citizens, but rather Assad loyalists who had killed rebel fighters and intimidated peaceful protesters.

Opposition activists filter most information about the rebels sent outside the country, making it hard to get an accurate picture.

An Associated Press reporter who spent two weeks with rebels in northern Syria in June found little evidence of rebel attacks on civilians, but the rebels were often merciless with regime troops and Assad loyalists. Some boasted freely about sending captured soldiers or loyalists deemed as collaborators "to Cyprus," which the rebels use as a euphemism for execution usually by gunfire.

Human rights groups have long documented reports of extrajudicial executions by people on both sides of the conflict, along with kidnappings, detentions, and widespread torture. As they gain confidence and more territory, however, rebel fighters appear to be increasingly resorting to these tactics.

In another video, posted Thursday, a group of about nine prisoners stand against a wall with their hands behind their backs, captured by rebels after they seized a police station in the embattled city of Aleppo. A rebel fighter says they would be put on trial in front of a Sharia (religious) court made up of "honorable judges."

The authenticity of the videos could not be independently verified.

The Syrian uprising began in March 2011 with largely peaceful protests but has since morphed into an insurgency and civil war. Activists say the conflict has already killed more than 19,000 people -- a figure which may soar with reports that foreign jihadists and extremists are streaming into Syria to join the fight against the regime.

The AP reporter counted almost two dozen rebel groups operating with little or no clear command structure. Some were more brutal than others. And there are doubtless countless of others who answer to nobody but themselves.

It is therefore difficult to call anybody to account for the increasingly ugly sectarian nature of Syria's conflict, where an opposition largely based among the country's Sunni majority has risen up against Assad's regime, which is dominated by members of the Alawite sect, an offshoot of Shiite Islam.

Many Iraqi Shiites, streaming back to their homeland in the past month to escape the war in Syria, reported a rash of attacks against their community, apparently by Sunni rebel gunmen.

In July alone, 23 Iraqi Shiites have been killed in Syria, some of them beheaded, according to the Washington-based Shiite Rights Watch. In one gruesome case, the U.N. said an Iraqi family of seven was killed at gunpoint in their Damascus apartment.

The motives for the attacks on Iraqis are unclear. They may be revenge against any Iraqi because the Shiite-led Iraqi government is seen as siding with Assad. They may also be fueled by sectarian hatreds, with resentment of Syria's Alawite leadership flaring into anger at Shiites.

Nevertheless, the rebels' main targets are not Iraqis, but rather Syrian security forces and regime loyalists. And the main battleground is now the country's largest city, Aleppo, where the new video of summary executions was shot.

According to activists, the executed prisoners were members of the powerful Barri clan, which has long had close ties to the Syrian government. Among them was the clan's leader, Ali Zinelabedine Barri, known as Zeino.

An earlier video shows Zeino sitting on the ground among a group of other prisoners, wearing only black underwear, with blood seeping from above one of his eye. The narrator says the prisoners had been terrorizing residents and earlier in the day killed 15 members of the "Brigade of Unification," the main group of rebel fighters in Aleppo.

A local activist who goes by the name of Abu Adel confirmed the executions, saying they took place on Tuesday at the courtyard of a school after major fighting between the FSA and members of the Barri clan in Aleppo's Bab al-Neyrab district.

The Barri clan, which Adel said numbers in the few thousands, were known to be hard-core regime loyalists and for months had intimidated residents who took part in peaceful anti-government protests. He said they were outlaws, drug dealers and "very well armed."

"Everyone in Aleppo hates them," Adel said, adding that they were given a "trial" before they were gunned down. "I wish it didn't happen, but the rebels are human after all. Sometimes it's hard to control people. We should understand their motives."

In a nod to criticism of rebel brutality, Abdel Razzaq Tlass, leader of the Farouk Brigade of the Free Syrian Army in central Homs province, pledged that his group would comply with Geneva conventions and treat captives according to international law.

"We are revolting against a barbarous regime that always tortured and treated detainees and arrestees in brutal ways that led to the death of many," Tlass said. "That's why we can never adopt the behavior of that very entity that we are revolting against."

Tlass made his pledge in a video that was posted on Monday -- a day before the Aleppo executions.


Associated Press writers Bradley Klapper and Ben Feller in Washington, Frank Jordans in Berlin and Bassem Mroue in Hatay, Turkey contributed to this report.