Libyan Observer wrote that gunmen attacked the headquarters of the National Oil Corporation (NOC) in Tripoli, repeating the scenario of last May's attack on the National Electoral Commission.
Reports say that about 6 gunmen broke into the building in the Dahra district, using small arms and grenades.
Two of the attackers were reportedly killed in a firefight with the security forces while two others blew themselves up, causing damage to the building.
The Special Deterrence Force said it was a terrorist attack by ISIS militants, publishing photographs of the body parts of two suicide bombers.
The Department of Homeland Security managed to evacuate all employees stranded on the roof of the NOC building, including President Mustafa Sanallah. There are also reports of casualties among NOC staff members.
Security forces smashed the windows so that the staff could escape. Several people have been injured by the shattered glass, witnesses said. NOC President Mustafa Sanalla exited the building's fire exit.
A staff member of a hotel near the NOC offices said he heard five explosions.
However, the NOC continued to function relatively normal throughout Libya, which relies on oil exports for most of its revenues.
Oil production has been hit by attacks on oil facilities and blockades, although it had partially recovered last year to extract around one million barrels per day (bpd).
Islamic militants have dormant cells in the northern cities and operate mobile units in the southern desert of Libya, according to Libyan and Western officials.
In May, the Islamic State claimed responsibility for a deadly attack on the offices of the National Electoral Commission in Tripoli. The group also claimed responsibility for an attack in 2015 at the Corinthia hotel, a landmark in Tripoli.