Update 5 at 2.15pm

Two staff of Libyan state oil firm and two gunmen were killed in an attack on the National Oil Corporation's (NOC) headquarters in Tripoli on Monday, a security official said.

Forces linked to the Tripoli government published pictures of a leg which it said belonged to one of the gunmen who had blown himself up.

"The death toll so far is two killed from NOC staff and two attackers," said Ahmad Ben Salim, spokesman for the Special Deterrence Force (Rada) force, which had surrounded the NOC building.

Gunfire could be heard as the security force allied to the Tripoli-based government arrived at the landmark building located in central Tripoli.

"Three or five gunmen were shooting inside the building," an NOC staff told Reuters after he said he had jumped out of a window to flee. "Several people were shot."

The NOC chairman, Mustafa Sanallah, was safely evacuated from the headquarters, a Reuters witness said, saying that he was seen being driven away in a car.

The NOC building is covered mainly in glass, and a witness said several people had been wounded by shattered windows.

Security force were smashing windows so staff could escape, witnesses said.

Smoke could be seen rising as security forces took up positions around the offices in central Tripoli and surrounding roads were cordoned off.

A Reuters witness saw ambulances leaving the site. A member of staff from a hotel next to the NOC offices said he had heard about five blasts.

Tripoli was shaken by clashes between rival armed groups at the beginning of this month but the capital has also seen occasional militant attacks.

In May, Islamic State claimed a deadly attack on the national election commission offices in Tripoli.

The group had also claimed an attack in 2015 on the Corinthia hotel, a landmark location in Tripoli.