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Libya's air force chief of staff said a fighter jet was shot down while carrying out airstrikes against Islamist militants on Monday.

Brig. Gen. Saqr al-Jaroushi told The Associated Press that a Libyan MIG32 was striking the positions of Islamic State group (IS) militants and other militias in the eastern town of Derna when it came under fire by anti-aircraft guns on Monday.

He said the pilot ejected and landed safely.

The al-Qaeda linked militant group Ansar al-Sharia claimed responsibility for the downing of the jet in a statement on Twitter.

The spokesman for a coalition of Islamist militias in Derna known as the Mujahidin Shura Council, in which Ansar al-Sharia is a member, Abdul-Moneim al-Shairy, confirmed that his group fired at the jet.

Four civilians were killed in Derna by an airstrike on Sunday, which al-Jaroushi says was carried out by a "," in a thinly-veiled reference to Egypt.

However, Egyptian army spokesman Mohamed Samir denied his country's forces carried out the strike.

IS and other Islamist militant groups have exploited the turmoil in Libya since the 2011 uprising that ousted longtime dictator Muammar Gaddafi to expand their influence in the country.

IS has established a stronghold in the , Gaddafi's hometown and a US defence official on Thursday said the number of IS fighters in , nearly double previous estimates.

The revelations come after the BBC reported that top IS commanders , as they come under increasing pressure from international airstrikes in Syria and Iraq.

Meanwhile, Libyan rivals are in the Moroccan resort of Skhirat in an attempt to agree on a unity government that would take charge of the country and restore order.

The talks have been fraught with difficulty as participants attempt to agree on a smaller cabinet after was rejected by the internationally recognised parliament.

Rivals now face the complicated task of accommodating the demands of the country's multiple political groups and militias.

Western powers including the US have indicated the  against IS in Libya, with reports suggesting the formation of a multi-national force to combat the terrorist group that is expanding close to European shores.

Agencies contributed to this report