TRIPOLI - Islamist fighters in the Fajr Libya (Libyan Dawn) coalition said on Saturday they have captured Tripoli's battered international airport after many days of clashes with nationalist militiamen.

The claim followed a setback the previous night when a mystery warplane raided Islamist positions, killing at least 10 fighters, a Fajr Libya spokesman said.

If independent sources confirm the airport has changed hands, it would be a major defeat for the nationalist fighters from Zintan west of Tripoli who have held the airport since the fall of long-time dictator Moamer Kadhafi in 2011. A statement shown on screen on An-Nabaa television, regarded as close to the Islamists, said: "Fajr Libya announces that it totally controls Tripoli international airport."

Earlier on Saturday, leaders of the Islamist coalition, partly comprising men from Misrata, east of Tripoli, said their forces were advancing on the airport, having taken a bridge and a military base.

The strategic site 30 kilometres (20 miles) south of the Libyan capital, has been shut since July 13 amid skirmishes between the Islamists and the Zintan force, allies of rogue general Khalifa Haftar, based at Benghazi in eastern Libya and hostile to the Islamists. The Islamist coalition, which repeatedly claims successes against the nationalists, on Thursday organised a visit by Libyan journalists to an army base on the way to the airport, to prove they had taken it.

Fajr Libya spokesman Mohamed al-Ghariani said on Saturday that an overnight air raid near the wreckage-strewn airport that killed at least 10 Islamists and wounded 20 was aimed at easing the pressure on the Zintan militia defending the hub.

- Mystery warplanes -

However, Ghariani said he could not identify the warplane that carried out the raid, just as two aircraft which bombarded Islamist positions on Monday night remain unidentified.

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 provenance 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, Ghariani said on An-Nabaa television.

Haftar 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 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.

Ghariani said the raids were undoubtedly aimed at relieving pressure on the Zintan militia.

The drawn-out battle for the airport has sparked the worst violence in the Libyan capital since the uprising.

Wild theories about the mystery aircraft abound in media and political circles. 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.

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 hired mercenaries and aircraft from an unknown army to launch the raids.