"It looks like a terrible attack," Trump told reporters at the White House.
"It would seem like it, based on the explosion. I met with some of our great generals and they just seemed to feel that it was," he added.
"This was not some kind of a manufacturing explosion type of event. It 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."
Lebanese officials have not described the explosions, which rocked the capital as an attack.
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."
Read also: Qatar to send field hospitals to support Beirut blast victims
General Security chief Abbas Ibrahim said "highly explosive material" that had been confiscated years earlier had been stored in the warehouse.
According to the Lebanese health ministry, at least 73 people were killed and 3,700 injured across wide parts of the country's biggest city.
Trump offered US assistance to Lebanon.
"Our prayers go out to all the victims and their families," he said. "The United States stands ready to assist Lebanon.
"We have a very good relationship with the people of Lebanon and we will be there to help."
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