The UN said Friday it had evacuated 163 refugees from war-ravaged Libya to neighbouring Niger, but more than 3,000 others were still trapped in detention centres affected by clashes.

The move marked the first evacuation of refugees and migrants out of Libya since fighting escalated in Tripoli two weeks ago, the UN refugee agency said.

United Nations Security Council diplomats began negotiations Tuesday on a British-drafted resolution that would demand an immediate ceasefire in Libya after forces loyal to commander Khalifa Haftar launched an offensive on Tripoli.

Egyptian President Abdel Fattah al-Sisi met Sunday with Libyan commander Khalifa Haftar, whose forces are fighting for control of the capital Tripoli, the presidency said.

The United Nations on Friday urgently appealed for the release and evacuation of more than 1,500 detained refugees and migrants caught in the crossfire of escalating fighting sparked by a self-styled Libyan army commander's military campaign to take the capital.

The U.N. refugee agency said the refugees and migrants are believed to be trapped in detention centers where hostilities are ranging.

More than 8,000 people have fled fighting around Libya's capital, half of whom have been displaced over the last two days, the United Nations said Friday. 

The UN chief warned Libya is on the brink of "a very dangerous situation" as forces loyal to the internationally recognised government and a rival strongman battle for control of the capital.

Thousands of people have fled heavy fighting on the outskirts of Tripoli that has left dozens dead and prompted mounting global alarm.

Fighting in Libya's capital has killed 56 people and wounded 266 over the last six days, the World Health Organization said Thursday, as the UN mobilised to support the country's over-stretched hospitals. 

"Thousands of people have fled their homes, while others are trapped in conflict areas. Hospitals inside and outside (Tripoli) are receiving daily casualties," the United Nations health agency said in a statement. 

The battle for Libya's capital intensified as the UN Security Council prepared to meet Wednesday to discuss the crisis gripping the North African country, where armed rivals are locked in a deadly power struggle.

The closed-door talks in New York come a day after the United Nations postponed a Libyan national conference aimed at drawing up an election roadmap because of fighting raging on Tripoli's doorstep.

Russia has said it is not taking sides in the fighting in Libya but behind the scenes, experts say, Moscow is firmly backing strongman Khalifa Haftar.

Russia on Sunday blocked a U.N. Security Council statement that would have called on forces loyal to Libyan commander Khalifa Haftar to halt their advance on Tripoli, diplomats said.