A warplane has attacked Tripoli's only functioning airport as eastern forces advancing on Libya's capital disregarded global appeals for a truce in the latest outbreak of warfare.
The fighting threatens to disrupt oil supplies, fuel migration to Europe and wreck UN plans for an election to end rivalries between parallel administrations in east and west.
Casualties are mounting.
The eastern Libyan National Army (LNA) forces of Khalifa Haftar - a former general in Muammar Gaddafi's army - said 19 of its soldiers had died in recent days as they closed in on the internationally recognised 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.
The UN said 2800 people had been displaced by clashes and many more could flee, though some were trapped.
"The United Nations continues to call for a temporary humanitarian truce to allow for the provision of emergency services and the voluntary passage of civilians, including those wounded, from areas of conflict," it said in a statement.
But that seemed to fall on deaf ears. Matiga airport, in an eastern suburb, said it was bombed and a resident confirmed the attack. No more details were immediately available.
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 towards the coastal capital.
Seizing Tripoli, however, is a much bigger challenge for the LNA. It has conducted air strikes on the south of the city as it seeks to advance along a road towards the centre from a disused former international airport.
However, the government of Prime Minister Fayez al-Serraj, 59, is seeking to block the LNA with the help of allied armed groups who have rushed to Tripoli from nearby Misrata port in utes fitted with machine guns.
UN envoy Ghassan Salame met Serraj in his office in Tripoli on Monday to discuss "this critical and difficult juncture", the world body's Libya mission said.
The violence has jeopardised a UN plan for an April 14-16 conference to plan elections and end anarchy that has prevailed since the Western-backed toppling of Gaddafi eight years ago.
The UN refugee agency expressed anxiety about thousands caught in crossfire and detention centres in conflict zones in a "rapidly deteriorating humanitarian situation".
As well as the UN, the EU, US and G7 bloc have all urged a ceasefire, a halt to Haftar's advance and return to negotiations.
Haftar casts himself as a foe of extremism but is viewed by opponents as a new dictator in the mould of Gaddafi, whose four-decade rule saw torture, disappearances and assassinations.