Casualties were mounting in fighting that also threatens to disrupt oil supplies, fuel migration to Europe and wreck UN plans for an election to end rivalries between parallel administrations in the country’s east and west.

The eastern Libyan National Army (LNA) forces of Khalifa Haftar - a former general in Gaddafi’s army - said 19 of its soldiers died in recent days as they closed in on the internationally recognized government in Tripoli.

A spokesman for the Tripoli-based Health Ministry said fighting in the south of the capital had killed at least 25 people, including fighters and civilians, and wounded 80, Reuters reported.

Mitiga airport, in an eastern suburb, was bombed and closed, authorities said. The UN envoy to Libya, Ghassan Salame, condemned the air strike as a “a serious violation of humanitarian law”.

A spokesman for the LNA confirmed the strike, saying his force had not targeted civilian planes, only a MiG parked at Mitiga.

The closure left Misrata airport, 200 km (125 miles) to the east down the coast, as the closest option for Tripoli residents.

Haftar’s LNA, which backs the eastern administration in Benghazi, took the oil-rich south of Libya earlier this year before advancing fast through largely unpopulated desert regions toward Tripoli.

Seizing the capital, however, is a much bigger challenge. The LNA has conducted air strikes on the south of the city as it seeks to advance along a road from a disused former international airport.

Witnesses said on Monday afternoon the LNA had lost control of the old airport and withdrawn from positions on the airport road. Forces allied to the Tripoli administration were seen inside the airport, while clashes with the eastern forces were raging south of the airport, a Reuters reporter at the scene and residents said.

On Sunday evening, LNA forces had moved up from the airport, coming as close as 11 km (7 miles) from the city center before retreating, residents said.