The blast appeared to have followed a fire that had broken out in the city’s port area, based on a video from the scene. The cause of the blast was immediately unknown. The force of the blast shook buildings, which were then hit again by the shock wave that blew out windows, sending shards of glass flying.

Lebanese Red Cross official Georges Kettaneh said there were dead and wounded, but did not have an exact figure, just saying there were hundreds dead or wounded.

Beirut’s governor Marwan Abboud called it a “national catastrophe” and the prime minister declared a day of mourning, according to CNN.

“It resembles to what happened in Japan, in Hiroshima and Nagasaki. That’s what [it] reminds me of. In my life, I haven’t seen destruction on this scale,” Abboud said of the mushroom cloud.

The White House and U.S. State Department said they are monitoring the situation.

“Having witnessed the horrific explosions at the port this evening, our heartfelt sympathies go out to the victims and their families. We mourn each loss from this terrible tragedy alongside the Lebanese people,” said U.S. Ambassador Dorothy Shea.

The cause of the blast was not immediately clear.