Lebanese rescue teams pulled out bodies and hunted for missing people on Wednesday from the wreckage caused by a massive warehouse explosion that sent a devastating blast wave across Beirut, killing at least 135.

Prime Minister Hassan Diab declared three days of mourning from Thursday as early investigations blamed negligence for the explosion at Beirut port, which has left tens of people missing and injured more than 5,000 others.

Up to a quarter of a million people were left without homes fit to live in, officials said, after shockwaves smashed building facades, sucked furniture out into streets and shattered windows miles inland.

The death toll was expected to rise from the blast, which officials blamed on a huge stockpile of highly explosive material stored for years in unsafe conditions at the port.

The explosion was the most powerful ever in Beirut, a city still scarred by civil war that ended three decades ago and reeling from an economic meltdown and a surge in coronavirus infections. The blast rattled buildings on the Mediterranean island of Cyprus, about 100 miles (160 km) away.

"No words can describe the horror that has hit Beirut last night, turning it into a disaster-stricken city," President Michel Aoun said in an address to the nation during an emergency cabinet session.

Aoun said 2,750 tonnes of ammonium nitrate, used in fertilisers and bombs, was stored for six years at the port after it was seized.

The government was "determined to investigate and expose what happened as soon as possible, to hold the responsible and the negligent accountable," he said.

An official source familiar with preliminary investigations blamed the incident on "inaction and negligence", saying "nothing was done" by committees and judges involved in the matter to order the removal of hazardous material.

The cabinet ordered port officials involved in storing or guarding the material to be put under house arrest, ministerial sources told Reuters.

Officials have not confirmed the origin of an initial blaze that sparked the explosion, although a security source and local media said it was started by welding work.

'COLLAPSE OF LEBANON'

For many, the blast was a dreadful reminder of the 1975-1990 civil war that tore the nation apart and destroyed swathes of Beirut, much of which had since been rebuilt.

Ordinary Lebanese, who have lost jobs and watched savings evaporate in the country's financial crisis, blamed politicians who have overseen decades of state corruption and bad governance.

"This explosion seals the collapse of Lebanon. I really blame the ruling class," said Hassan Zaiter, 32, a manager at the heavily damaged Le Gray Hotel in downtown Beirut.

Relatives gathered at a cordon to Beirut port seeking information on those still missing as the search continued. Many of those killed were port and custom employees, people working in the area or those driving nearby during the Tuesday evening rush hour. Some victims were hurled out to sea by the powerful blast.

The Red Cross was coordinating with the Health Ministry to set up morgues as hospitals were overwhelmed. Health officials reported that hospitals were running out of beds and equipment to attend to the injured.

Beirut's Clemenceau Medical Center was "like a slaughterhouse, blood covering the corridors and the lifts," said Sara, one of its nurses.

Beirut Governor Marwan Abboud told Al Hadath TV that collective losses from the blast might reach as high as $15 billion, including indirect losses related to business.

"This is the killer blow for Beirut, we are a disaster zone," said Bilal, a man in his 60s, in the downtown area.

The World Bank Group said on Wednesday it would work with Lebanon's partners to mobilize public and private financing for reconstruction and recovery. It was unclear what effect the disaster would have on the country's difficult negotiations with the International Monetary Fund, however.

Offers of international support poured in.

Gulf Arab states, who in the past were major financial supporters of Lebanon but recently stepped back because of what they say is Iranian meddling, sent planes with medical equipment and other supplies.

Turkey said it would send 20 doctors to help treat the injured, as well as medical and relief assistance. Iraq pledged fuel aid, while Iran offered food and a field hospital.

Iranian Supreme Leader Ayatollah Ali Khamenei tweeted: "We sympathize with the dear Lebanese citizens and stand by them in the painful tragedy of the Beirut port explosion ... Patience in the face of this incident will be a golden leaf of honour for Lebanon."

The United States, Britain and other Western nations, which have been demanding political and economic change in Lebanon, also offered aid. Germany, the Netherlands and Cyprus offered specialised search and rescue teams.

Two French planes were expected to arrive on Thursday with specialist rescue personnel and equipment, and President Emmanuel Macron was due to visit on Thursday.

FOOD SECURITY

"This is a catastrophe for Beirut and Lebanon," Mayor Jamal Itani told Reuters while inspecting damage.

The port district was left a tangled wreck, disabling the nation's main route for imports needed to feed a nation of more than 6 million people.

The country's main grain silo at the port was destroyed in the blast and Beirut Governor Abboud said a crisis might develop without international intervention.

Lebanon was already struggling to house and feed refugees fleeing conflict in neighbouring Syria and has no trade or other ties with its only other neighbour Israel.

"On a scale, this explosion is scaled down from a nuclear bomb rather than up from a conventional bomb," said Roland Alford, managing director of British explosive ordnance disposal firm Alford Technologies. "This is huge."

The blast also prompted the Special Tribunal for Lebanon on Wednesday to postpone its verdict in the trial over the 2005 bombing that killed ex-Prime Minister Rafik al-Hariri to Aug. 18. The tribunal's decision had been expected this Friday.

The U.N.-backed court put on trial four suspects from the Iranian-backed Shi'ite Muslim group Hezbollah. Hariri and 21 others were killed by a big truck bomb in another area of the Beirut waterfront, about 2 km (about one mile) from the port.

White House Chief of Staff Mark Meadows said the U.S. government had not totally ruled out the possibility that Tuesday's explosion was an attack, and said it is still gathering intelligence on the blast.