UN Secretary General Antonio Guterres has called for an "immediate halt" to military operations in Libya after fighting displaced thousands of people and Tripoli's only functioning airport was attacked.
Guterres called for de-escalation and the prevention of "all-out conflict," spokesman Stephane Dujarric said in a statement.
He said "there is no military solution to the Libya conflict" and called on all parties to engage in immediate dialogue.
A warplane attacked the Libyan capital's only functioning airport on Monday as eastern forces advanced from the south, ignoring appeals for peace.
The fighting threatens to disrupt oil supplies, prompt migration to Europe and wreck UN plans for an election to end rivalries between parallel administrations in east and west. Casualties are also 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 Tripoli government.
A spokesman for the Tripoli-based Health Ministry said fighting in the south of the capital had killed at least 25 people and wounded 80. The UN said 2800 people had been displaced by clashes.
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 larger challenge. The LNA has conducted air strikes on the south of the city as it seeks to advance along a road towards the centre from a disused 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 in utes fitted with machine guns.
UN envoy Ghassan Salame met Serraj in his office in Tripoli on Monday to discuss the situation, 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 ex-leader Gaddafi eight years ago.
Haftar portrays his campaign as a "war on terrorists" but his opponents accuse him of seeking to crush dissent and establish a military dictatorship.
With Reuters
