"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 said.
"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 fertilizer 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."