Lebanon's prime minister has called for a day of mourning on Wednesday after a huge explosion killed at least 100 people and injured more than 4,000 in Beirut.
Hassan Diab said his country was experiencing "a real catastrophe" and said those responsible would pay the price.
He tweeted that Tuesday's blast was caused by more than 2,700 tonnes of ammonium nitrate left unsecured at a warehouse near the city's port for six years.
The head of Lebanon's Red Cross, George Kettaneh, said at least 100 people were killed and more than 4,000 were wounded, adding the toll could rise further.
Video shows the moment a building exploded, sending a huge shockwave across the city that was heard and felt as far away as Cyprus - more than 125 miles (200km) away.
Pictures showed cars upturned and streets covered in shattered glass and twisted metal, with homes and businesses reduced to rubble.
Lebanon's internal security chief, Abbas Ibrahim, also said the blast may have been caused by explosive material confiscated from a ship and stored at the port.
Earlier reports had suggested the building was a fireworks storage warehouse.
US President Donald Trump said his generals believed the explosion was probably a bomb attack - but did not provide any evidence or further explanation.
Residents reported collapsed ceilings and shattered windows as far away as 2km (1.2 miles), as a huge column of red smoke rose over the city.
On Wednesday morning smoke was still rising from the port, as residents woke to a scene of devastation.
Images showed the wounded - bloodied and dazed - walking in the streets, calling for help or in search of missing loved ones.
Hospitals - many severely damaged and without power due to the blast and already battling the coronavirus pandemic - were struggling to cope with the influx of casualties.
Wards and corridors were overflowing, forcing medics to treat some of the injured in the rubble outside.
One hospital said it could not take any more patients and was appealing for blood, local media said, as others pleaded for generators to keep their lights on.
Sky News Middle East editor Zein Ja'far, who was in downtown Beirut at the time of the explosion, said it had caused windows to "cave in".
"It tore apart the facade of the building we're in, and once the dust settled we managed to get ourselves and others in this block outside," said Ja'far.
Fady Roumieh was standing in a car park east of the blast and said it was "like a nuclear bomb".
"The damage is so widespread and severe all over the city," said the Beirut resident.
"Some buildings as far as 2km are partially collapsed. It's like a war zone. The damage is extreme. Not one glass window intact."
As night fell, a fire was still active in the port district and ambulance sirens sounded across the city.
Schools minister Nick Gibb said all embassy staff based in Beirut were accounted for, but it is not yet known how many British nationals are among those caught up in the aftermath of the disaster.
Prime Minister Boris Johnson said the government is "ready to provide support in any way we can", while the Foreign Office said it is "monitoring the situation closely".
"There have been a small number of non-life threatening injuries and we are investigating the effect on British nationals in Beirut, and our embassy is ready to supply consular support for our nationals who are affected by this devastating and appalling tragedy," Mr Gibb told Sky News.
He confirmed that the Lebanese prime minister had asked the UK for help and that ministers and officials were urgently working out what support can be offered to the country.
Israel has said it had "nothing to do with the incident" and humanitarian and medical assistance is being offered as "this is the time to transcend conflict".
The explosion comes as Lebanon experiences its worst economic and financial crisis in decades - and amid rising tensions between Israel and militant Hezbollah group along Lebanon's southern border.