Government forces and helicopters belonging to a Libyan renegade general bombed ammunition sites of suspected Islamist militants in the eastern city of Benghazi, a military commander and residents said.

Islamist forces have been trying to take the civilian and military airport from government forces in the port city, a confrontation forming part of a broader picture of anarchy in the North African country three years after the ouster of Muammar Gaddafi.

Western powers and Libya's neighbours fear the oil producer will turn into a failed state. A weak government is unable to control former rebels who helped topple Gaddafi but now fight each other for power.

Wanis Bukhamada, commander of army special forces in Benghazi, told Reuters his forces had attacked with artillery several ammunition stores in camps held by Islamists late on Wednesday.

Residents also heard helicopters and huge blasts lighting up the night sky in a Benghazi suburb. On Thursday war planes could be also heard.

Benghazi, home of headquarters of state oil firms, has been a war zone since renegade army general Khalifa Haftar declared war in May on Islamists roaming the area unchallenged.

He has allied himself with the special forces but despite that alliance, Islamists have managed to overrun several army camps in Benghazi.

Libya's government and elected House of Representatives has relocated to the remote eastern city of Tobruk after a separate armed group seize the capital Tripoli and most government institutions last month.

The new Tripoli rulers have set up a rival parliament and government not recognized by the international community.