Prime Minister Scott Morrison confirmed on Wednesday morning one Australian citizen was among at least 70 people reportedly killed by the explosion in a port warehouse district near the centre of the city, which injured more than 3,500 others.

The blast sent shockwaves across the city, shattering windows and causing apartment balconies to collapse.

"We obviously can't confirm details of that at this stage because there is contacts with families an others," Mr Morrison told Nine's Today Show.

"But our hearts go out to all of those in Lebanon and in Beirut in particular at the moment. You can see from the image of the blast, it is just absolutely devastating."

He said all staff at the Australian embassy in Beirut were safe but the building had been "significantly compromised".

Around 20,000 Australians are generally living in Lebanon, Mr Morrision said, although it was not known how many returned to Australia due to the pandemic.

In a tweet, Mr Morrison said Australia " stands ready to provide our support, including to any Australians affected."

Foreign Affairs Minister Marise Payne said she was "shocked and saddened" by the horrific event.

"Our hearts go out to all affected and to all Lebanese Australians worried for loved ones today."

A phoneline has been set up for anyone in need of urgent help, on +61 2 6261 3305.

Officials expected the death toll to rise sharply as emergency workers dug through rubble across a swathe of the city to rescue people and remove the dead.

It was the most powerful blast to hit Beirut in years, making the ground tremble.

"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 - in all the streets and areas near and far from the explosion."

Three hours after the blast, which struck shortly after 6pm local time on Tuesday, a fire still blazed in the port district, casting an orange glow across the night sky as helicopters hovered and ambulance sirens sounded across the capital.

A security source said victims were being taken for treatment outside the city because Beirut hospitals were already packed with wounded.

Red Cross ambulances from the north and south of the country and the Bekaa valley to the east were called in to cope with the huge casualty toll.

The blast was so big that some residents in the city, where memories of heavy shelling during the 1975 to 1990 civil war live on, thought an earthquake had struck.

Dazed, weeping and wounded, people walked through streets searching for relatives.

Lebanon's interior minister said initial information indicated highly explosive material, seized years ago, that had been stored at the port had blown up.

The minister later told al-Jadeed TV ammonium nitrate had been in storage there since 2014.

Footage of the explosion shared by residents on social media showed a column of smoke rising from the port district followed by an enormous blast, sending a ball of white smoke and fireball into the sky.

Those filming the incident from high buildings 2km from the port were thrown backwards by the shock.

Lebanese President Michel Aoun called for an emergency meeting of the country's Supreme Defence Council, according to the presidency's Twitter account.

Prime Minister Hassan Diab called for a day of mourning on Wednesday.

It was not immediately clear what caused Tuesday's blaze that set off the blast.

Internal Security Chief Abbas Ibrahim, touring the port area, said he would not pre-empt investigations.

An Israeli official said Israel, which has fought several wars with Lebanon, had nothing to do with the blast.

The governor of Beirut port told Sky News that a team of firefighters at the scene had "disappeared" after the explosion.

"I saw a fireball and smoke billowing over Beirut. People were screaming and running, bleeding. Balconies were blown off buildings. Glass in high-rise buildings shattered and fell to the street," said a Reuters witness.

In Cyprus, a Mediterranean island 180km across the sea from Beirut, residents heard the blast bangs.

One resident in Nicosia said his house and window shutters shook.

- with AAP