Paramilitary forces established a base on December 17 in the formerly safe city of Wad Madani in war-torn Sudan, sending thousands fleeing, many of them already displaced.
Thousands have sought to escape the former aid hub since the war between the Army and the paramilitary Rapid Support Forces (RSF) reached Wad Madani on Friday, according to the United Nations.
But they faced prohibitive costs and other hurdles.
Previously, the city had been "one of Sudan's few remaining sanctuaries", according to the Norwegian Refugee Council's Sudan director William Carter.
Echoing the brutal urban warfare in the capital Khartoum, 180 kilometres north, fighter jets flew overhead as the sound of explosions echoed across the Wad Madani, according to an AFP correspondent who said the RSF had set up a base in the city's east.
American Ambassador John Godfrey urged the RSF to "cease their advance" on Al-Jazirah state, where nearly half a million displaced people had sought refuge, and to "refrain from attacking" state capital Wad Madani.
"A continued RSF advance risks mass civilian casualties and significant disruption of humanitarian assistance efforts," Mr. Godfrey said in a statement on December 17.
Regional bloc IGAD's executive secretary Workneh Gebeyehu said he was "extremely concerned by the resurgence of conflict" and called for the cessation of hostilities.
Wad Madani alone houses more than 86,000 displaced people, according to figures from the UN, which has suspended all humanitarian field missions in Al-Jazirah state.
More than 2,70,000 of the city's 7,00,000 residents had been dependent on humanitarian aid, the UN said.
The war between army chief Abdel Fattah al-Burhan and his former deputy, RSF commander Mohamed Hamdan Daglo, has destroyed the country's already fragile infrastructure, economy and health care system.
By early December, it had killed at least 12,190 people, 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.
Families scrambled on December 17 to once again flee to safety but found bus tickets had quadrupled to $60 a head, and many had nowhere to go.
"A continuous flow of people, many of them who already ran for their lives just a few months ago, are now rushing towards already heavily burdened and resource-depleted cities in neighbouring states," the NRC's Carter said.
"We are also extremely worried for highly vulnerable families in Wad Madani who have been crammed into displacement sites in schools for months and have nowhere to hide from violence, no means to escape and nowhere else to flee," Carter added.
Sudan's doctors' union said on December 17 the situation in the city has become "catastrophic" after pharmacies were forced shut.
Both forces have been accused of indiscriminate firing on residential areas, as well as targeting civilians, activists and healthcare personnel.
