US defence officials reject Trump's claims that Beirut explosion was caused by 'a bomb'

US defence officials have said there is no evidence to back President Donald Trump’s claims that a massive explosion in Beirut which killed at least 100 people was a deliberate act.

The three experts told CNN that if there was any indication that the blast was caused by a bomb, as the president claimed, then there would be movements of American troops in the region.

So far, there has been no increase in protection of US interests or bases, the experts said.

Another unnamed defence official said they "didn't know what Trump was on about", according to CNN, when he claimed the blast was due to "an attack".

Trump spoke to reporters after two explosions were reported in the Lebanese capital's port area on Tuesday afternoon.

The second blast caused huge devastation in surrounding areas and knocked out windows in neighbourhoods across the city.

Lebanese officials immediately rejected claims that the blast was caused to air strikes or a bombing, as some initially suggested, saying the tragedy was likely due to an industrial accident.

Later in the day, Lebanon's head of general security said the incident was caused by "high explosive materials" but did not elaborate.

It seems highly likely that the blast was caused by the storage of hundreds of tons of ammonium nitrate which had been sitting in the port for years.

Trump offered his sympathies to the Lebanese people and offered help but said the incident "looks like a terrible attack".

When asked to elaborate, Trump continued with the theory that the blast was caused by a bomb, despite there being no evidence of this.

"It would seem like it based on the explosion," Trump said. "I've met with some of our great generals and they just seem to feel that it was not a - some kind of manufacturing explosion type of event," he said.

"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."

The entire port has been devastated by the blast and at least 100 killed with the death toll expected to rise.

Tens of thousands were also injured with massive damage to homes and offices.

Prime Minister Hassan Diab said 2,750 tonnes of the agricultural fertiliser ammonium nitrate that had been stored for years in a Beirut portside warehouse had blown up, sparking "a disaster in every sense of the word."

"What happened today will not pass without accountability," said Diab. "Those responsible for this catastrophe will pay the price."