An attack on an airbase in south Libya has killed 141 people, mostly soldiers loyal to military strongman Khalifa Haftar, reports confirmed on Friday.

An attack on an airbase in south Libya has killed 141 people, mostly soldiers loyal to military strongman Khalifa Haftar, a spokesman for his forces said on Friday.

Members of the Third Force militia loyal to the UN-backed Government of National Accord in Tripoli on Thursday attacked the base used by Haftar's self-proclaimed Libyan National Army (LNA), military sources said earlier.

LNA spokesman Ahmad al-Mesmari said the victims included civilians who worked at the Brak al-Shati airbase or were in the nearby area, also reporting that summary executions took place.

"The soldiers were returning from a military parade. They weren't armed. Most of them were executed," he said.

The GNA said in a statement late on Friday that a commission of inquiry had been set up to investigate the attack.

It said it had decided to suspend its defence minister, al-Mahdi al-Barghati, and the head of the Third Force from their duties until those responsible were identified.

Both the GNA and defence ministry earlier condemned the assault and said they had not ordered any such action.

The unity government, the rival administration in eastern Libya and their respective backers are battling for influence in the north African country which has been wracked by chaos since the fall of dictator Muammar Gaddafi in 2011.

The LNA does not recognise the authority of the GNA, and instead supports the rival authorities based in the east.

Earlier on Friday, the UN envoy to Libya voiced alarm at reports of the attack on the base, 650 kilometres south of Tripoli.

"I am outraged by reports of significant numbers of fatalities, including civilians and by reports that summary executions may have taken place," UN envoy Martin Kobler said in a statement.

Britain's ambassador to Libya, Peter Millett, also denounced the assault.

"Disgusted by attack on Brak al-Shati & reports of mass executions. Perpetrators must be brought to justice," he wrote on Twitter.