Lebanon is in mourning following a huge explosion in the capital Beirut killed 100 people and injured more than 4,000.
The blast sent seismic shockwaves through the Lebanese capital, destroying dozens of buildings and shattering windows across the city on Tuesday evening.
President Michel Aoun said there had been 2,750 tonnes of ammonium nitrate that was unsafely stored in a warehouse at the port where the blast took place.
An investigation is under way to find the exact trigger for the explosion and Lebanon's Supreme Defence Council said those responsible would face the "maximum punishment”.
Lebanon: Beirut explosion aftermath - In pictures
President Aoun called for an emergency cabinet meeting on Wednesday and said a two-week state of emergency should be declared.
"What we are witnessing is a huge catastrophe," the head of Lebanon's Red Cross George Kettani told broadcaster Mayadeen. "There are victims and casualties everywhere.”
Dramatic footage of an explosion shared on social media showed a column of smoke rising from the port, followed by an enormous blast, sending up a white cloud and a fireball into the sky.
People sought their missing loved ones in the overflowing hospitals.
One medic said 200 to 300 people had been admitted to a single emergency department. "I've never seen this. It was horrible," the medic, who gave her name as Rouba, said.
Prime Minister Hassan Diab told the nation there would be accountability for the deadly blast at the "dangerous warehouse", adding "those responsible will pay the price."
Donald Trump said US military generals had told him that they "seem to feel" the massive explosion was a "terrible attack,” likely caused by a bomb.
The US leader was asked why he called it an attack and not an accident, especially since Lebanese officials say they have not determined the cause of the explosion.
He told reporters at the White House: "It would seem like it based on the explosion. I met with some of our great generals and they just seem to feel that it was.
“This was not a — some kind of a manufacturing explosion type of a event. ... They seem to think it was an attack. It was a bomb of some kind, yes."
Health minister Hamad Hasan earlier confirmed that at least 78 people were killed and nearly 4,000 injured.
The death toll has now reached 100 and more victims are still under the rubble, the head of the Lebanese Red Cross told local broadcasters.
George Kettaneh told LBCI TV that the Red Cross was coordinating with the health ministry for morgues to take victims because hospitals were overwhelmed.
The explosion occurred three days before a UN-backed court is due to deliver a verdict in the trial of four suspects from Hezbollah over a 2005 bombing which killed former Prime Minister Rafik al-Hariri and 21 others. Hariri was killed by a huge truck bomb on the same waterfront, about 2 km (about one mile) from the port.
Israeli officials said Israel, which has fought several wars with Lebanon, had nothing to do with Tuesday's blast and said their country was ready to give humanitarian and medical assistance.
Shi'ite Iran, the main backer of Hezbollah, also offered support, as did Tehran's regional rival Saudi Arabia, a leading Sunni power.
Qatar and Iraq said they were sending makeshift hospitals to assist the high numbers of casualties.
The US, Britain, France and Germany expressed shock and sympathy and said they were read to help.
UK prime minister Boris Johnson confirmed that British nationals were among those caught up in the aftermath of the blast.
Mr Johnson said the Government is "ready to provide support in any way we can", and Foreign Secretary Dominic Raab said the UK stands in solidarity with Lebanon.
The blast threatens a new humanitarian crisis in a nation that hosts hundreds of thousands of Syrian refugees and which is already grappling with economic meltdown under one of the world's biggest debt burdens.
Images showed port buildings reduced to tangled masonry, devastating the main entry point to a country that relies on food imports to feed its population of more than six million.
Residents said glass was broken in neighbourhoods on Beirut's Mediterranean coast and inland suburbs several miles away.
In Cyprus, a Mediterranean island 110 miles across the sea from Beirut, residents heard the blast.