A huge fire raged in Beirut port on Thursday, sparking alarm among Lebanese still reeling from the devastating dockside explosion that disfigured the capital last month.

It was not immediately clear what caused the blaze just over a month after the August 4 blast which killed more than 190 people, wounded thousands and ravaged much of the capital.

Huge columns of black smoke, visible from faraway neighbourhoods, billowed above the site of the fire.

Haitham, a 33-year-old worker at a company at the port, told AFP how he fled the new fire in fear.

"We were working when all of a sudden they started yelling at us to get out," he said. "There was welding going on... and a fire broke out. We don't know what happened.

"We dropped everything and started running ... It reminded us of the explosion."

The interim head of the port, Bassem al-Kaissi, told Lebanese television channel LBC that the blaze started in the port's free zone, where an importer had stocked cooking oil containers and tyres.

The fire "started with oil containers before moving on to the tyres," he said. "It was either caused by the heat or by a mistake. It's too early to say."

The army reported it was responding to the fire, also saying it had broken out at a warehouse containing oil and tyres.

"Operations have begun to extinguish the fire and army helicopters will take part," it said on Twitter.

Social media users posted video footage which unsettled Beirut residents only just recovering from the country's deadliest peacetime disaster.

"Insane fire at the port, causing a panic all across Beirut. We just can't catch a break," Aya Majzoub, a researcher for the group Human Rights Watch, wrote on Twitter.

"We can't take this much trauma," another user wrote.

The August 4 blast sparked widespread outrage after it emerged authorities had been aware for years of the presence of the huge stockpile of ammonium nitrate that blew up, and the scandal forced the government to resign.

Criminology researcher Omar Nashabe tweeted about the latest disaster: "Where are we living?"

"This is the scene of the crime a month ago! Where is the judiciary? Where is the state? Where is responsibility?"

The port blast had heaped new misery on Lebanese already battling the coronavirus pandemic and the country's worst economic crisis in decades, which has seen poverty rates double to more than half the population.

Lebanon has launched a probe into the blast, one off the biggest non-nuclear explosions ever, and arrested 25 suspects so far.

Among them are top port and customs officials, as well as Syrian workers who allegedly carried out welding hours before the explosion.

Lebanon has rejected an international investigation into the explosion, but its probe is being aided by foreign experts, including from the American FBI and France.