Sudan's army is trying to hold off attacks by the Rapid Support Forces near Wad Madani, a major city in a region that had been sheltered from fighting and taken in hundreds of thousands of civilians during eight months of war.
Clashes in the area, about 170km southeast of Khartoum, threaten to open a new front in a conflict that has displaced nearly seven million people, left the capital in ruins and triggered waves of ethnically driven killings in Darfur.
A takeover of densely populated Wad Madani, capital of El Gezira state and a hub for aid operations, could cause massive displacement and deepen a humanitarian crisis in which the United Nations is warning of famine-like conditions in areas directly affected by the conflict.
The UN said all humanitarian field missions in El Gezira state, an important agricultural region, were suspended until further notice.
The International Organisation for Migration (IOM) said the clashes had led to "widespread displacement" in affected areas, and that the situation remained "tense and unpredictable".
El Gezira's governor declared a night-time curfew, Sudan's state news agency reported.
On social media, some shared photos of people packing up belongings while others asked about safe routes out of Wad Madani, which has an estimated population of 700,000, of whom more than 270,000 need humanitarian aid.
Residents in Khartoum and other cities have reported rape, looting, arbitrary killing and detention by RSF forces.
The RSF, a paramilitary force that has recently gained momentum with takeovers of major cities, has said it is trying to protect civilians.
It said in a statement on Friday that it was seeking to destroy army strongholds and that citizens in Wad Madani and El Gezira state should be reassured.
Witnesses told Reuters that RSF fighters had reached Um Eleila, 15km from Wad Madani, but that the army had deployed on a bridge that separates the two areas and army planes were seen overhead.
Witnesses in Wad Madani said they could hear explosions but there was no fighting inside the city.
Um Eleila town contains fuel depots that could aid the RSF in its campaign.
The air force and artillery had inflicted major losses on the RSF and blocked an incursion towards the city, an army source told Reuters.
The war between the army and the RSF began in Khartoum on April 15.
The two forces shared power with civilians after a popular uprising in 2019 and seized control in a coup in 2021, then fell out over plans for a new political transition.
Internationally backed attempts to mediate between the warring sides, both of which say they can win an outright victory, have so far proven fruitless.
More than 12,000 people have died, according to the United Nations.
Nearly 1.5 million have fled Sudan and more than 5.4 million have been internally displaced, according to the IOM, making Sudan the country with the highest number of displaced people in the world.
Almost 500,000 of those had fled to El Gezira state, including about 86,400 in Wad Madani.
