The paramilitary Rapid Support Forces on Sunday set up a base near Wad Madani, where their offensive has sent thousands fleeing Sudan's second city and former aid hub, many of them already displaced. The paramilitaries have continued to press deeper into the city, witnesses say.

UN Secretary General Antonio Guterres "is deeply concerned about reports of fighting between the Sudanese armed forces and the Rapid Support Forces in the vicinity of Wad Madani in Al-Jazira state," said his spokesman Stephane Dujarric.

"The city has served as a hub for humanitarian operations since the start of the conflict, and had not been directly impacted by the conflict until this recent round of fighting."

Since fighting broke out on April 15 between army chief Abdel Fattah al-Burhan and his former deputy, RSF commander Mohamed Hamdan Daglo, the city of Wad Madani, 180 kilometers (110 miles) south of Khartoum, became a haven for thousands of displaced people during the conflict.

The United Nations Office for the Coordination of Humanitarian Affairs (OCHA) said in a statement that the city's population had reached 700,000, after more than half a million people took shelter there, among them 270,000 who need humanitarian assistance.

"The continued escalation of violence in Sudan is devastating the country, as well as the region," Dujarric told a media briefing.

"We reiterate our call to the Sudanese armed forces as well as the Rapid Support Forces to immediately cease fighting.

"If the fighting continues, aid distribution to two million people -- that's about a third of the state's population -- will be compromised."

The US embassy in Khartoum warned on Sunday in a statement that "the continued advance of the Rapid Support Forces threatens massive civilian casualties and significant disruption to humanitarian assistance efforts."