The president said the explosions – which killed more than 70 people and injured thousands of others – "looked like an attack" despite no immediate evidence suggesting the blasts were intentional.
Lebanese officials have not publicly determined the cause of Tuesday's explosions but have pointed to a warehouse that was improperly storing thousands of tonnes of unsecured ammonium nitrate for several years.
"I met with some of our great generals and they just seem to feel that it was [an attack]," 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."
US Department of Defence officials said there was no immediate indication that the explosions were the result of an attack, according to CNN. The Pentagon has also referred requests for comment to the White House.
Brett McGurk, a former national security official in the Trump administration as well as under former presidents Barack Obama and George W Bush, said the president's remarks were "wildly irresponsible".
"It's wildly irresponsible for a president to stand at the [White House] podium and spitball about an international incident like this as hundreds of casualties are still missing or being treated," he said on Twitter.
He added that the Defence Department "should clean this up tonight".
Walter Shaub, a former director of the US Office of Government Ethics, added that the US should "avoid speculation and wait until we have information from a reliable source".
"The President of the United States of America is not a reliable source."
Lebanese General Security chief Abbas Ibrahim said the explosions were likely set off by material that had been seized several years ago, according to the Associated Press.