Lebanon's outgoing premier Hassan Diab said Tuesday that an FBI investigation into an August 4 explosion at the Beirut port found it was caused by 500 tonnes of ammonium nitrate. 

The prime minister, who resigned in the wake of the blast that killed more than 200 people, had previously said that more than 2,700 tonnes of ammonium nitrate fertiliser had been stored haphazardly at a port warehouse for years. 

But a report by the US domestic intelligence agency, which aided Lebanon in the probe, said that less than a quarter of that amount had exploded, Diab told reporters during a briefing. 

"The FBI report revealed that the amount that exploded is only 500 tonnes," he said. "Where did the (other) 2,200 tonnes go?" he asked.

AFP could not independently verify the contents of the FBI report. 

Nearly five months after the blast, little light has been shed on the circumstances that led to Lebanon's worst peacetime disaster, which is widely blamed on decades of negligence and corruption by the country's ruling elite.

The slow pace of the investigation has sparked outrage at home and fuelled distrust among international donors, whose support is much needed if Lebanon is to stand a chance of surviving its deepest economic crisis in decades.

Lead investigative judge Fadi Sawan this month charged Diab and three former ministers over the explosion in the first set of indictments against politicians.   

He charged them with "negligence and causing death to hundreds and injuries to thousands more" in the first such official indictment against a prime minister in office in Lebanese history.

The blast probe has since been suspended after two of the charged ministers called on Sawan to be replaced. Lebanon's top Court of Cassation must rule on their request before investigations proceed. 

The investigation had led to the arrest of at least 25 suspects, including the chief of the port and its customs director, but not a single politician.