What happened?

An initial large explosion in the port area of Beirut took place around 6:00pm, resulting in a fire, several small blasts and then a colossal explosion that flattened the harbour front and surrounding buildings. Seismologists measured the event, which blew out windows at the city's international airport nine kilometres (more than five miles) away, as the equivalent of a 3.3-magnitude earthquake.

Why such a big blast?

Lebanon's Prime Minister Hassan Diab said 2,750 tonnes of ammonium nitrate, an agricultural fertiliser, stored in a portside warehouse had blown up. Ammonium nitrate is an odourless crystalline substance that has been the cause of numerous industrial explosions over the decades.  When combined with fuel oils, it creates a potent explosive widely used in the construction industry, but also by insurgent groups such as the Taliban for improvised explosive devices. Lebanon's General Security chief Abbas Ibrahim said the material had been confiscated years earlier and stored in the warehouse, located close to Beirut's shopping and nightlife districts.

Was it deliberate?

There has been no indication from Lebanese officials that the explosions were caused deliberately. US President Donald Trump said late Tuesday that US generals had told him the explosions appeared to have been caused by a "bomb of some kind." But a Pentagon spokesman, when asked about the president's remarks, told AFP that "we don't have anything for you" and "you will have to reach out to the White House for clarification."

How many casualties?

The blasts killed more than 100 people and injured over 4,000, the Lebanese Red Cross said. Injuries were recorded right across the city, with glass blown out of buildings in multiple districts. Lebanon's national defence council has declared Beirut a disaster zone.