In Pics: Devastating images of Beirut Blast

A large blast in the Lebanese capital, Beirut, has killed at least 70 people and injured more than 4,000.

Videos show smoke billowing from a fire, then a mushroom cloud following the blast at the city's port.

Officials are blaming highly explosive materials stored in a warehouse for six years.

President Michel Aoun tweeted it was "unacceptable" that 2,750 tonnes of ammonium nitrate was stored unsafely.

An investigation is under way to find the exact trigger for the explosion. Lebanon's Supreme Defence Council said those responsible would face the "maximum punishment" possible.

Hospitals are said to be overwhelmed and many buildings have been destroyed.

President Aoun declared a three-day mourning period, and said the government would release 100 billion lira (£50.5m; $66m) of emergency funds.

A BBC journalist at the scene reported dead bodies and severe damage, enough to put the port of Beirut out of action.

The streets were chaotic in the moments after the explosion.

Prime Minister Hassan Diab called it a catastrophe and said those responsible must be held to account.

He spoke of a "dangerous warehouse" which had been there since 2014, but said he would not pre-empt the investigation.

Local media showed people trapped beneath rubble. A witness described the first explosion as deafening, and video footage showed wrecked cars and blast-damaged buildings.

"All the buildings around here have collapsed. I'm walking through glass and debris everywhere, in the dark," one witness near the port told AFP news agency.

The blast was heard 240km (150 miles) away on the island of Cyprus in the eastern Mediterranean.

The explosion comes at a sensitive time for Lebanon, with an economic crisis reigniting old divisions. Tensions are also high ahead of Friday's verdict in a trial over the killing of ex-Prime Minister Rafik Hariri in 2005.

Lebanon's prime minister also called for international help: "I make an urgent appeal to friendly and brotherly countries... to stand by Lebanon and to help us heal our deep wounds," Hassan Diab said.

UK Prime Minister Boris Johnson tweeted: "The pictures and videos from Beirut tonight are shocking. All of my thoughts and prayers are with those caught up in this terrible incident.

US President Donald Trump sent his deepest sympathies after what he called "a terrible attack", and his Secretary of State Mike Pompeo offered assistance, tweeting: "We are monitoring and stand ready to assist the people of Lebanon as they recover from this horrible tragedy."

France said it was sending aid and resources to Lebanon.

Iran would "render assistance in any way necessary" Foreign Minister Mohammad Javad Zarif tweeted, while Saudi Arabia expressed its full solidarity with Lebanon.

Israel said in a statement that it had "approached Lebanon through international security and diplomatic channels and has offered the Lebanese government medical and humanitarian assistance".

The German foreign ministry said the blast had been felt at its embassy in the city.

"We cannot for the moment exclude German nationals figuring among the dead and wounded," it said in a statement. Meanwhile, on Twitter Pray for Beirut was trending.

Shocking images from Beirut blast the blast have gone viral on social media