A Libyan warplane from forces loyal to the internationally recognised government has bombed a Greek-operated oil tanker anchored off the coast, killing two crewmen amid an escalation of conflict between factions vying to rule the country.

Military officials said the vessel had acted suspiciously after a warning not to enter port and said it was suspected of transporting Libyan fighters to Derna, the eastern port city where the ship was at anchor when it was hit on Sunday.

But the state-run oil firm NOC said it had leased the ship to carry fuel for power generation to Derna from Brega, an oil port to the west. The vessel was damaged, but none of the 12,600 tonnes of heavy oil leaked out, the Athens-based operator Aegean Shipping Enterprises Co said.

Greece condemned the "unprovoked and cowardly" attack that killed one Greek and one Romanian crew member and wounded two others and said it had contacted the UN envoy for Libya and the European Union about the incident.

"The Greek government will take all the necessary actions towards Libyan authorities, despite the unrest, so that light is shed on the tragic incident, the attackers identified and punished and the families of the victims reimbursed," it said.

The strike on the Liberian-flagged vessel ARAEVO was part of increasingly chaotic violence in Libya which has two parallel governments: the officially recognised one, which has been pushed out of the capital, and the administration run by a faction known as Libya Dawn that seized Tripoli last summer.

Conflict over control

Each side has appointed its own officials to run NOC and the oil ministry, leading to confusion over who controls what.

After a special meeting of the Arab League in Cairo, the head of Libya's elected parliament called on Arab states to intervene to protect the country's oil installations.

Fighting for control of oil assets has slashed Libya's oil output to 380,000 barrels per day from the 1.6 million bpd

produced before a civil war ousting of Muammar Gaddafi in 2011.

The NOC office in Tripoli, where the rival government is now in control, said it had informed authorities of the vessel's plans. It said the incident would impact "negatively" on incoming tankers to Libya's ports

But Ahmed Bu Zayad Al-Mismari, a spokesman for the internationally recognised government's general chief of staff, said: "The NOC in Tripoli did not inform us...The tanker may have been involved with terrorists or it may have been taken over at sea by terrorists so that is why we bombed it."

Since the war that ended Gaddafi's four-decade rule, rival nationalist, Islamist, tribal and regionalist forces have battled for power. But the conflict has coalesced around two loosely aligned factions.

The government and elected parliament which have been pushed out of Tripoli to Tobruk, an eastern port town some 150 km (100 miles) from the Egyptian border, has allied itself with ex-rebel forces in Zintan, near Tripoli, and a former Gaddafi army general, Khalifa Haftar, who is conducting a campaign against armed groups.

Tripoli is now controlled by a self-declared government set up by forces allied to the city of Misrata, reinstating a former parliament and taking over ministries.