International aid flights began arriving on Wednesday as Lebanon's leaders struggled to deal with the widespread damage and shocking aftermath of the blast.

At least 135 people were killed and about 5000 injured.

Public anger mounted against the ruling elite being blamed for the chronic mismanagement and carelessness that led to the disaster.

The Port of Beirut and customs office is notorious for being one of the most corrupt and lucrative institutions in Lebanon where various factions and politicians, including Hezbollah, hold sway.

The investigation is focusing on how 2750 tons of ammonium nitrate, a highly explosive chemical used in fertilisers, came to be stored at the facility for six years, and why nothing was done about it.

Losses from the blast are estimated up to $US15 billion ($A21 billion), Beirut Governor Marwan Abboud told Saudi-owned TV station Al-Hadath, adding that nearly 300,000 people are homeless.

Hospitals were overwhelmed by the injured. One damaged in the blast had to evacuate all patients to a nearby field for treatment.

It was the worst single explosion to strike Lebanon, a country whose history is filled with destruction - from a 1975-1990 civil war, conflicts with Israel and periodic terrorist attacks.

Lebanon already was on the brink of collapse amid a severe economic crisis and the coronavirus pandemic.

A senior US Defense Department official and member of the US intelligence community said there were no indications the explosion was the result of an attack by either a nation state or proxy forces.

They told AP the explosion seems to have been caused by improper storage of explosives.

Fueling speculation negligence was to blame, an official letter circulating online showed the head of the customs department had repeatedly warned the huge stockpile of ammonium nitrate at the port was a danger and had asked for a ruling on a way to remove it.

Badri Daher confirmed to local LBC TV there were five or six such letters. He said his predecessor also pleaded with the judiciary to issue orders to export the explosive materials.

Daher said it was his duty to "alert" authorities of the dangers but that is the most he could do.

The 2750-ton cargo had been stored at the port since it was confiscated from a ship in 2013 and on Tuesday it is believed to have detonated after a fire broke out nearby.

President Michael Aoun vowed the investigation would be transparent and those responsible punished.

Cabinet has ordered an unspecified number of Beirut port officials put under house arrest pending the inquiry.

The government also said public schools and some hotels will be opened for the homeless and promised unspecified compensation for the victims.

Australian Associated Press