The UN has called for safe passage for trapped civilians out of the Sudanese city of el-Fasher after RSF announced they had seized control of the army's main base there.

Sudan's military has not acknowledged loss of the site, which would be a significant victory for the Rapid Support Forces (RSF) in the ongoing civil war.

UN chief Antonio Guterres said the latest fighting marked a "terrible escalation" in the conflict, adding that the suffering of civilians was "unbearable", AFP news agency reports.

El-Fasher is the last army foothold in the vast western region of Darfur, and has been besieged by the RSF and its allies for 18 months.

Heavy fighting has been reported since Saturday after RSF fighters captured the home of the North Darfur governor.

Social media videos verified by the BBC now show RSF combatants celebrating the capture of the army's el-Fasher headquarters.

The city is one of the worst battlegrounds of Sudan's civil war, leading the UN to call it an "epicentre of suffering".

The UN's top humanitarian official Tom Fletcher said he was deeply alarmed at the reports of civilian casualties.

"With fighters pushing further into the city and escape routes cut off, hundreds of thousands of civilians are trapped and terrified - shelled, starving, and without access to food, healthcare, or safety," Fletcher said in a statement.

"Civilians must be allowed safe passage and be able to access aid," he added.

The US has also called for safe passage and is trying to negotiate a ceasefire.

But it is likely a sign that the civil war will continue, not end.

Sudan has been ravaged by conflict since 2023, after top commanders of the RSF and Sudanese army fell out and a vicious power struggle ensued.

More than 150,000 people have died across the country and about 12 million have fled their homes, creating one of the worst humanitarian crises.

The army controls most of the north and the east, with el-Fasher being until now the last major urban centre in Darfur still held by government forces and its allies.

The RSF controls almost all of Darfur and much of the neighbouring Kordofan region.

The group has previously said that it hopes to form a rival government in al-Fasher when it assumes complete control.