At leaders 250,000 people have fled fighting in a Sudanese city once seen as a safe haven for those escaping the civil war, the UN said.

The paramilitary Rapid Support Forces (RSF) - who are fighting the army - have advanced into Wad Madani.

For months, the city has housed large numbers who had sought safety from the fighting in the capital, Khartoum.

People in Wad Madani, south-east of Khartoum, are "scattering in different directions", an aid worker said.

"People are fleeing with nothing more than the clothes on their back with nowhere really to turn to, to go to," Will Carter, the Norwegian Refugee Council's country director for Sudan told the BBC's Newsday programme.

A doctor from Wad Madani, in the process of leaving the city with his family, told the BBC by text: "We are looking for a safe place. None of the Wad Madani citizens will be able to talk to you now.

"Imagine how it feels when you don't have any place to go, when all your years, your work, your glory, is gone within seconds. It is hard to speak."

Residents of the city said the RSF had attacked a hospital and had taken over an army base.

There is international concern that the war - which began in April - is spreading.

Over the weekend, the US State Department called on the RSF to cease its advance on Wad Madani, which is also a major aid hub.

Washington said the group's actions were inconsistent with its stated aim of protecting Sudanese civilians.

There are also reports of renewed fighting in the city of Nyala in Sudan's western Darfur region.