U.S. defence officials contradict Trump's claim of Beirut 'attack' as president doubles down

U.S. President Donald Trump on Wednesday afternoon continued to suggest that the massive explosion that killed at least 135 people in Lebanon might not have been an accident, as Lebanese officials have initially assessed.

"Whatever happened, it's terrible, but they don't really know what it is," Trump said. "Nobody knows yet."

Investigators probing the deadly blast that ripped across Beirut are focused on possible negligence in the storage of tonsof ammonium nitrate, a highly explosive fertilizer, in a waterfront warehouse. The Lebanese government ordered the house arrest of several port officials in connection with the explosion.

Earlier Wednesday, U.S. officials saidthere is no indication the massive explosion was an attack, contradictingTrump, who said in the first hours after the Tuesday blastthat American generals told him it was likely caused by a bomb.

Defence Secretary Mark Esper, asked about the situation while appearing remotely at the Aspen Security Forum, said the United Stateswas "still getting information on what happened."

"Most believe it was an accident as reported, and beyond that I have nothing further to report on that," he said.

Earlier, two officials speaking only on condition of anonymity to discuss internal assessmentstold The Associated Press that while it was not out of the realm of possibility that Tuesday's blast in Beirut was deliberately caused, the belief so far is that it was most likely an accident.

From the outset, U.S. officials have said that they did not know the cause of the initial fire and explosions that set off the larger blast. But they saidthey do believe the reports out of Lebanon claiming a large stockpile of ammonium nitrate left over from a seizure is what exploded.

'A terrible attack'

On Tuesday, Trump offered condolences to the victims and said the U.S.stood ready to assist Lebanon.

"It looks like a terrible attack," he said.

Trump was asked why he called it an attack and not an accident, especially since Lebanese officials said they had not determined the cause of the explosion.

He 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 seem to feel that it was. This was not a some kind of a manufacturing explosion type ofevent.... They seem to think it was an attack. It was a bomb of some kind, yes."

Esper did not address who Trump was referring to when citing conversations with generals.

Trump chief of staff defends comments

Also Wednesday, Trump's chief of staff, MarkMeadows, defended Trump's description of the explosion to reporters in an interview with CNN.

"I can tell you the initial report was exactly what the president shared with all of you. I happen to know that he was briefed on that. The initial reports looked at an explosion," he said. "We still have not totally ruled that out."

At least 1 American killed

From video and other evidence, experts suggested that fireworks and ammonium nitrate were the fuel that ignited the explosion that rocked the Lebanese capital.

The scale of the damage from the area of the explosion at the port of Beirut to the windows blown out kilometresaway resembled other blasts involving the chemical compound commonly used as an agricultural fertilizer.

The compound typically does not detonate on its own and requires another ignition source. That likely came from a fire that engulfed what initially appeared to be fireworks stored at the port.

The Lebanese government said it was putting an unspecified number of Beirut port officials under house arrest pending an investigation into how approximately 2,750 metrictonnes of ammonium nitrate came to be stored at the port for years.

"It's obviously a tragedy," Esper said. "You know, we mourn for the dozens, if not hundreds, of Lebanese possibly killed and thousands hurt.... When you see the video, it's just devastating."

Esper said the U.S. was preparing to provide humanitarian aid and medical or other supplies to the Lebanese people.

The U.S. Embassy in Beirut said at least one American citizen was killed and several more were injured in the explosion.

"We are working closely with local authorities to determine if any additional U.S. citizens were affected," the embassy said in a statement Wednesday. The embassy said all of its employees are safe and accounted for.

Secretary of State Mike Pompeo spoke Wednesday with Lebanese Prime Minister Hassan Diab to convey U.S. condolences to the Lebanese people, according to State Department deputy spokesperson Cale Brown.