Tripoli (AFP) - Using the cover of darkness to mask its identity, a mystery warplane launched a raid near Tripoli airport overnight, killing at least 10 Islamist, a Libyan militia spokesman said Saturday.

The new strike came five days after an initial night raid alarmed Tripoli residents and sparked questions, yet to be answered, about where the aircraft had come from.

Neither the targeted militants nor the Libyan government, lacking real power and holed up in Tobruk 1,600 kilometres (1,000 miles) east of Tripoli, have been able to shed light on the identity of the two planes in Monday's raid.

The latest deadly strike targeted an army base to the south of Tripoli and a nearby warehouse, Mohammed al-Ghariani, spokesman for the Fajr Libya (Libyan Dawn) coalition of militias, said on An-Nabaa television.

Ghariani said "at least 10 men" from Fajr Libya died and 20 were wounded in the raid, a follow-up to Monday's initial air bombardment.

"We haven't identified the plane that carried out the raid, just like those that attacked us on Monday," the Fajr Libya spokesman said.

The Islamist fighters, including a group from Misrata, east of Tripoli, had recently seized the army base struck in the raid from a rival militia hailing from Zintan, west of the capital.

During a lull in the fighting, the Islamist militia organised a visit on Thursday for Libyan journalists to prove they had captured the building, which was the army's headquarters during the regime of long-time dictator Moamer Kadhafi, overthrown in 2011.

- Airport shut -

Rogue general Khalifa Haftar, who opposes the Islamists and favours the Zintan militia, claimed to be behind Monday's raid, but specialists doubted his ability to carry out such at attack.

An air force unit which has refused to join an offensive launched by Haftar in the eastern city of Benghazi said the aircraft were "foreign, not Libyan".

It said Libyan aircraft are not equipped to make night flights and cannot be refuelled in flight, particularly if they take off from remote air bases controlled by Haftar's forces.

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Ghariani said the raids were undoubtedly aimed at relieving pressure on the Zintan militia, who, he said, were struggling to fend off Fajr Libya's campaign aimed ultimately at taking control of Tripoli international airport.

The hub has been shut since July 13 amid the repeated skirmishes, the worst violence in the Libyan capital since the uprising.

Media and political circles are full of the wildest theories about the mystery aircraft. Some speculate about Western intervention, but France, Italy and the United States have all denied involvement.

The Islamists do not rule out foreign aircraft acting at the behest of the Libyan government, after the new parliament elected on June 25 called for foreign intervention to protect civilians.

Others suggest the trail leads towards neighbouring countries, in particular Egypt, where new President Abdel Fattah al-Sisi is hostile towards Islamists. Authorities in Cairo have refused to comment.

Algeria to the immediate west has repeatedly said it will not get involved in Libya.

Prime Minister Abdelmalek Sellal said on August 7: "A consensus has to be reached to put in place a government and institutions capable of running" Libya.

But "going in with our forces to restore the situation is not a solution and cannot be a solution," he said.

Meanwhile, some sources in Libya say Haftar has acquired Sukhoi jets from Russia capable of carrying out such raids.

Another theory is that the embattled Zintan fighters have hired mercenaries and aircraft from an unknown army to launch the raids.