Trump says nobody can say now whether Beirut explosion was an attack

US President Donald Trump said on Wednesday that nobody can say whether a devastating explosion in Beirut could have been caused by an attack, casting doubt on investigators who have blamed negligence for the warehouse blast that killed at least 135 people.

More than 5,000 other people were injured in Tuesday's explosion at Beirut port and up to 250,000 were left without homes fit to live in after shockwaves smashed building facades, sucked furniture out into streets and shattered windows miles inland.

US Defence Secretary Mark Esper said on Wednesday that the devastating explosion was an "accident".

"Most believe that it was an accident, as reported, and beyond that, I have nothing further to report on. It's obviously a tragedy," Esper said during a virtual interview at the Aspen Security Forum on Wednesday.