'It was a bomb': Trump says deadly Beirut explosion was 'terrible attack' – despite no evidence it was deliberate

Lebanese officials believe explosions were caused by thousands of tonnes of ammonium nitrate left unsecured for years in warehouse

Donald Trump has described deadly explosions in Lebanon as a "terrible attack" before officials have determined the cause of the blasts and with no immediate evidence to suggest they were intentional.

Massive explosions in Beirut on Tuesday have killed at least 70 people and injured more than 3,000 others while levelling a large segment of the capital.

Multiple videos showed fires and thick plumes of smoke emerging from a building before a mushroom cloud swallows the area.

"Let me begin by sending America's deepest sympathies to the people of Lebanon," the president said at a White House briefing on Tuesday. "It looks like a terrible attack."

Lebanese officials believe the explosions were caused by thousands of tonnes of ammonium nitrate left unsecured for several years in a warehouse.

Conspiracy theories quickly spread on social media following the explosions as experts and officials urged caution against rampant speculation within a country reeling from several crises.

When asked what evidence supported his characterisation, the president claimed that US generals told him they believed the explosions were the result of an attack.

"I met with some of our great generals and they just seem to feel that it was," the president said. "This was not some kind of a manufacturing explosion type of event. This was a – seems to be, according to them, they would know better than I would – but they seem to think it was an attack. It was a bomb of some kind, yes."

The US Geological Survey reported that the explosions were as powerful as an earthquake with a magnitude measuring 3.3 on the Richter scale.

Lebanese Prime Minister Hassan Diab said that "those responsible will pay for what happened" following an investigation into the "dangerous conditions" at the warehouse.

"I promise you that this catastrophe will not pass without accountability," he said in televised remarks.

Lebanese officials have called for international aid to help heal "deep wounds" in the aftermath.

On Twitter, UK Prime Minister Boris Johnson said: "The pictures and videos from Beirut tonight are shocking. All of my thoughts and prayers are with those caught up in this terrible incident.

"The UK is ready to provide support in any way we can, including to those British nationals affected."

US Secretary of State Mike Pompeo said that the US is "monitoring" and "ready to assist the people of Lebanon as they recover from this horrible tragedy".

During the president's briefing, Joe Biden – his Democratic rival – said that "our hearts and prayers are with the people of Lebanon, and the victims of the horrific explosion in Beirut".

"I urge both the Trump Administration and international community to immediately mobilise assistance to the thousands injured in the blast," he said.