The UN Security Council has demanded that Sudanese paramilitary forces call off their eight-week siege of el-Fasher, a city in the Darfur region where fighting has sparked fears of a possible genocide.

Sudan's army has been fighting the paramilitary Rapid Support Forces (RSF) for more than a year, in a civil war that has killed thousands and forced millions from their homes.

El-Fasher is the last major urban centre in Darfur that remains in the hands of Sudan's army.

The security council has called for "an immediate halt to the fighting" and withdrawal of all troops from the city.

The 15-member council on Thursday adopted a British-drafted resolution, with 14 votes in favour, while Russia abstained.

It expressed “grave concern” at the spreading violence and credible reports that the RSF are carrying out “ethnically motivated violence” in el-Fasher.

In a statement, the council called on the rival forces to “to seek an immediate cessation of hostilities, leading to a sustainable resolution to the conflict, through dialogue”.

The resolution called on all parties to allow civilians who wish to leave el-Fasher to do so and remove obstacles to humanitarian access.

Britain's UN envoy Barbara Woodward told the council that "an attack on the city would be catastrophic for the 1.5 million people sheltering in the city".

"This council has sent a strong signal to the parties to the conflict today. This brutal and unjust conflict needs to end," she added.

Human Rights Watch's Louis Charbonneau said the resolution "puts the Sudanese Armed Forces and Rapid Support Forces on notice that the world is watching".

The security council also urged member states to "refrain from external interference," and demanded compliance with the arms embargo on the country.

El-Fasher's last functioning hospital has been forced to close down after an attack on the facility.

More than 130,000 residents have fled the city due to fighting between April and May, the UN said.

UN experts warn that the Darfur region is facing a growing risk of genocide as the world's attention is focused on conflicts in Ukraine and Gaza.

Rights groups in Darfur have accused the RSF of using rape as a weapon of war, and is targeting darker-skinned Masalit people and other non-Arab groups in a campaign of ethnic cleansing.

But the RSF says it is not involved in what it describes as a "tribal conflict" in Darfur.

Several rounds of peace talks have failed to end the war, which began when the two generals leading the army and RSF respectively fell out.