Hours after a blaze began to destroy the Notre Dame Cathedral in Paris, French President Emmanuel Macron vowed to a mourning nation: "We will rebuild."

The cathedral is still engulfed in flames and was seen billowing smoke over the Parisian skyline Monday night. The cathedral's iconic 300-foot spire collapsed in the blaze, but firefighting officials believe the two main bell towers will survive. Macron said Monday night "the worst has been avoided."

The cause of the massive fire is not immediately known, although authorities are saying that after a preliminary investigation the cause is believed to be accidental. The fire began shortly before 7 p.m. Monday, starting in the attic and ultimately enveloping almost the entirety of the monument.

About 400 firefighters are combating the blaze, with one firefighter reportedly seriously injured.

Addressing a largely American audience, French Ambassador to the United States Gérard Araud spoke of the damage done to his country's pride. "I can't think of the skyline of Paris without a spire of Notre Dame, you know, which has just collapsed. It's 1,000 years of my history, of — it's our national identity, which is burning," Araud said on CNN.

[Opinion: The US should pay 17.83% of Notre Dame's reconstruction costs]

The cathedral is one of the most well-known monuments in the world, attracting millions of visitors every year. Construction on the medieval cathedral began in the 12th century. In the 1790s, the building suffered much damage during the French Revolution, but a 25-year restoration of the historic monument began in 1845.

The cathedral is also in the heart of Paris or, "kilometer zero," the spot from where the distance of all French cities from Paris is measured.

President Trump tweeted about the fire shortly after 1:30 p.m. EST, writing: "So horrible to watch the massive fire at Notre Dame Cathedral in Paris. Perhaps flying water tankers could be used to put it out. Must act quickly!"

He also discussed the disaster at a stop in Minnesota later in the afternoon, saying it was a “terrible sight to behold" and “there’s probably no cathedral in the world like it. It’s a terrible scene."

Trump later followed up with a tweet that said: "God bless the people of France!"

Paris Mayor Anne Hidalgo urged onlookers to respect the perimeter that has been set up around the area of the cathedral, where people in Paris were seen Monday evening singing hymns.

The spire of the famous cathedral was undergoing a $6.8 million renovation at the time of the fire, and local media has quoted the Paris fire brigade as saying the cause is “potentially linked” to the work.

First lady Melania Trump expressed her condolence over the tragedy Monday afternoon, tweeting: "My heart breaks for the people of Paris after seeing the fire at Notre Dame Cathedral. Praying for everyone’s safety."

Ivanka Trump, the president's daughter and senior adviser, called the blaze a "devastating and shocking sight" and said she was praying for those in Paris.

Vice President Mike Pence said: "Notre Dame is an iconic symbol of faith to people all over the world – and it is heartbreaking to see a house of God in flames. Our thoughts and prayers are with the firefighters on the scene and all the people of Paris."

Bells in churches across France tolled in solidarity with the Notre Dame as one of the most iconic cathedrals in the world continues to smolder. Further causing heartache, the fire began during Holy Week — the most sacred week in Christianity.