UN Secretary-General Antonio Guterres on Monday strongly condemned the military escalation near Tripoli and called for an immediate halt to the fighting in Libya.
The appeal followed an air attack by the forces of commander Khalifa Haftar on the Mitiga airport east of the capital.
Guterres "urges the immediate halt of all military operations in order to de-escalate the situation and prevent an all-out conflict," said a UN statement.
He "strongly condemns the military escalation and ongoing fighting in and around Tripoli, including the aerial attack today by a Libyan National Army (LNA) aircraft against Mitiga airport."
The air strike shut down Tripoli's only functioning airport as fighting raged around the capital and thousands fled.
Haftar's self-styled LNA claimed Monday's air strike against the airport, with a spokesman saying the attack targeted a MiG-23 military plane and a helicopter.
Haftar launched the offensive on Tripoli last week just as Guterres was in Libya to push for a political deal on holding elections.
A UN-backed unity government controls the capital, but its authority is not recognized by a parallel administration in the east of the country.
As fighting escalated over the weekend, the United Nations called for a humanitarian pause to allow civilians trapped in the violence to escape, but the appeal fell on deaf ears.
Libya has been rocked by violent power struggles between an array of armed groups since the NATO-backed overthrow of dictator Moamer Kadhafi in 2011.
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