An attack on an airbase in southern Libya has killed at least 141 people, mostly soldiers loyal to renegade General's Khalifa Haftar's self-proclaimed Libyan National Army (LNA), a spokesman for his forces said.

LNA's Ahmad al-Mismari told the AFP news agency on Friday that members of a militia loyal to the UN-backed Government of National Accord (GNA) in Tripoli had carried out summary executions during Thursday's assault.

The victims also included civilians who worked at the Brak al-Shati airbase or were in the nearby area, Mismari said.

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

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The GNA denied ordering the attack, which it condemned, and said it had set up a commission of inquiry to investigate the allegations.

Mahdi al-Barghathi, the GNA's defence minister, was suspended, pending an outcome of the probe. Jamal al-Treiki, the head of GNA's Third Force militia, which was accused of carrying out the attack, was also suspended.

Libya has been wracked by chaos since the 2011 uprising that toppled and killed longtime leader Muammar Gaddafi, with rival militias and authorities vying for control of the oil-rich country.

The LNA does not recognise the authority of the GNA, and instead supports rival authorities based in the east. It has promised a "strong" response to the attack.

Martin Kobler, the UN envoy to Libya, voiced "outrage" at reports of deaths.

He suggested that if the reports were true, the "unprovoked attack" could amount to war crimes and may be prosecuted by the International Criminal Court.

Aguila Saleh, the speaker of the Tobruk-based parliament in eastern Libya, accused the GNA's Third Force of a "serious breach" of a truce agreement reached between Haftar and Fayez al-Sarraj, head of the Tripoli-based government, in Abu Dhabi on May 2.

Following the reconciliation talks, both men said they had agreed to put an end to violence in southern Libya, where tribes and militias vie for control of lucrative smuggling routes with neighbouring Chad, Niger and Sudan.

Saleh said he has ordered the armed forces "to take the measures necessary to respond to the assault and defend the south and cleanse it of all outlaw militias".

His comments prompted alarm in Tripoli, with the GNA calling for an immediate ceasefire in the south.