BEIRUT — A "colossal" explosion that rocked the port area of Lebanon's capital on Tuesday, killing dozens and injuring thousands, happened at a warehouse where tons of ammonium nitrate was being stored, the country's prime minister said.

"I will not rest until we find the person responsible for what happened, to hold him accountable and impose the most severe penalties," Prime Minister Hassan Diab said.

Diab said it was "unacceptable" that a shipment of ammonium nitrate estimated at 2,750 tons had been in warehouse for six years without "preventive measures" in place to protect it. The chemical compound, which is commercially available, is used widely in fertilizers and explosives.

It wasn't clear what ignited the shipment, but at least 3,000 people were injured and 50 were killed, Minister of Health Hassan Hamad said at a press conference earlier Tuesday. Those numbers are likely to rise with hospitals filling up fast.

Images and videos on social media appear to show large plumes of smoke and damaged buildings.

Image: Beirut explosion (AFP - Getty Images)

The head of Lebanon's General Security agency, Maj. Gen. Abbas Ibrahim, who toured the Beirut port to inspect the damage, said: "It is not possible to pre-empt investigations and say that there was a terrorist act."

The National News Agency also reported that wheat was being stored in a nearby warehouse. Firetrucks were en route to the scene and evacuation operations were underway, it said. Military and security personnel were also trying to ease traffic to make way for emergency vehicles.

The Lebanese Red Cross also confirmed on Twitter that it had more than 30 teams, including ambulances, responding to the blast and had put out an urgent call for blood donations.

"Injuries are definitely within the hundreds," a spokesman for the Red Cross told NBC News by telephone. "I cannot give a specific number of casualties at the moment, we are overwhelmed with cases."

Lebanese President Michel Aoun said on Twitter that the Ministry of Health would meet the expenses of the treatment for the wounded, and that the government would provide shelter and support to displaced families whose properties were damaged in the blast.

French Foreign Minister Jean-Yves Le Drian offered condolences and said the country was ready to offer assistance to its former colony.

Image: Beirut explosion (Anwar Amro / AFP - Getty Images)

'Lots of destruction'

Footage from the Daily Star, a Lebanese newspaper's offices in the city, appear to show parts of the roof fallen in, blown-out windows and damaged furniture, indicating the strength of the blast.

An NBC News journalist in the city felt the explosion from her apartment, more than a kilometer from the port, as her windows and doors were blown out, filling the stairwell with thick dust as residents dashed to exit the building.

Dozens of residents, some covered in blood, scrambled to their cars to leave the area for the safety of nearby mountains, a population accustomed to catastrophe.

"It's like Hiroshima," Beirut Mayor Jamal Itani told reporters.

"There is lots of destruction and the wounded are lying in the streets."

Another NBC News journalist said the blast was "colossal" and could be felt miles away as it rippled through the capital, leaving a trail of destruction behind.

In an interview with NBC News, Aly Sleem, 34, who was driving in a neighborhood near the explosion when it occurred, said he heard what sounded like two blasts.

“The second one was really horrible,” he said. “You couldn't even breathe.”

Image: Beirut explosion (Anwar Amro / AFP - Getty Images)

A journalist who recorded the second explosion, Ahmad Yassine, 30, captured what appeared to be a mushroom cloud rising from the building.

Yassine was having lunch a few kilometers away at the time, and  in a video that he tweeted, a loud blast followed by screams can be heard moments after the cloud dissipates.

At first, Yassine said in an interview, he didn’t know if he was still breathing.

“Later on, I saw people running, destruction, glass everywhere,” he said.