Fighting between the Sudanese army and paramilitaries engulfed the aid hub of Wad Madani Friday triggering an exodus of civilians already displaced by eight months of war, an AFP correspondent reported.
When war broke out in April between the Sudanese armed forces and the paramilitary Rapid Support Forces (RSF), Wad Madani quickly became a safe haven for civilians fleeing heavy fighting in the capital Khartoum, 180 kilometres (110 miles) to the north.
Nearly half a million displaced people sought refuge in Al-Jazirah state, more than 86,000 of them in its capital Wad Madani, according to United Nations figures.
But after fighting overran the city on Friday, the UN humanitarian agency OCHA announced it had ordered "a suspension of all humanitarian field missions within and from Al-Jazirah state... until further notice".
More than 270,000 of the city's 700,000 residents had been dependent on humanitarian aid, the UN said.
An AFP correspondent reported that fighting had spread into the city from the northeast after the regular army pounded paramilitaries in its northern outskirts with air strikes earlier in the day.
In a bleak reminder of the first days of the war in Khartoum, shops and businesses in Wad Madani were quickly boarded shut, the AFP correspondent reported.
Civilians fled on foot with anything they could carry, desperate to secure transport to safer areas to the south or east.
The war between army chief Abdel Fattah al-Burhan and his former deputy, RSF commander Mohamed Hamdan Daglo, has claimed more than 12,190 lives, according to a conservative estimate by the Armed Conflict Location and Event Data Project.
Over 5.4 million people are internally displaced, while about 1.3 million have fled abroad, according to UN figures.
RSF fighters had been advancing towards Wad Madani for weeks, down the highway from Khartoum.
In a statement, the RSF sought to "assure dear citizens" in Al-Jazirah and Wad Madani that "the goal of our forces is to destroy the strongholds" of the army.
The United States in early December said the rival forces have both committed war crimes.
Secretary of State Antony Blinken said the RSF has also carried out ethnic cleansing and crimes against humanity. He pointed to accounts of mass killings by the largely Arab force and its allied militias against the ethnically African Masalit people in Sudan's western Darfur region.