Libya announced on Sunday a state of emergency in and around the capital Tripoli, amid escalating violence between warring militias.
An estimated 39 people, including civilians, have been killed and 96 wounded in the ongoing clashes.
The government in a statement called on rival militias to stop the fighting and abide by the UN-brokered ceasefire.
On Friday the only operational airport in Tripoli was forced to suspend all flights following an escalation in violence.
The spokesman for the UN Secretary General Antonio Guterres, Stephane Dujarric, condemned the clashes, "in particular, the use by armed groups of indiscriminate shelling leading to the death and injury of civilians, including children."
In a statement the UN reminded all parties that the indiscriminate use of force is a violation of international humanitarian and human rights law and urged "all parties to grant humanitarian relief for those in need, particularly those who are trapped by the fighting."
Thousands of migrants are among those left isolated and without access to food because of the clashes.
The UN refugee agency said it moved about 300 refugees and migrants out of a detention centre in Ain Zara, one of the focal points for conflict.
The UN-backed government in Tripoli tried to bring about a ceasefire and instructed two of its most senior military commanders to mediate between the militias.
Maj Gen Osama Juwaili, the western zone commander, said: “All of the fighting forces must return to the headquarters they held before the clashes.”
The fighting pits pro-government militias against a brigade known as the Seventh, or the Kani, and allied forces led by Salah Badi, a militia leader who was blamed for leading the destruction of Tripoli in 2014. He has seemingly returned from his base in Turkey.
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