Fighting between Sudan's military and a notorious paramilitary group forced up to 300,000 people to flee their homes in a province that had been a safe haven for families displaced by the devastating conflict in the northeastern African country, the U.N. said Thursday.

The fighting erupted in the city of Wad Medani, the provincial capital of Jazeera province, after the Rapid Support Forces attacked the city earlier this month. The RSF said that it took over Wad Medani earlier this week, and the military said that its troops withdrew from the city, and an investigation was opened.

Sudan’s war began in mid-April after months of tensions between military chief Gen. Abdel-Fattah Burhan and RSF commander Gen. Mohammed Hamdan Dagalo. Both generals led a military coup in October 2021 that derailed Sudan’s short-lived transition to democracy following a popular uprising that forced the removal of President Omar al-Bashir in April 2019.

The U.N. agency International Organization for Migration said that between 250,000 and 300,000 people fled the province — many reportedly on foot — to safer areas in the provinces of al-Qadarif, Sinnar and the White Nile. Some sheltered in camps for displaced people and many sought shelter in local communities, it said.

Jazeera, Sudan’s breadbasket, was home to about 6 million Sudanese. Since the war, about 500,000 displaced fled to the province, mostly from the capital, Khartoum, which has been the center of fighting, according to the U.N. Office for the Coordination of Humanitarian Affairs. Medani, which is about 100 kilometers (60 miles) southeast of Khartoum, had hosted more than 86,000 of the displaced, OCHA said.

The World Food Program announced Wednesday that it has temporarily halted food assistance in some parts of Jazeera, in what it described a “major setback” to humanitarian efforts in the province.

The U.N. food agency said that it had provided assistance to 800,000 people in the province, including many families that fled the fighting in Khartoum.

The conflict in Sudan has wrecked the country and killed up to 9,000 people as of October, according to the United Nations. However, activists and doctors’ groups say the real toll is far higher.

Catherine Russel, executive director of the U.N. children's agency, UNICEF, warned Thursday that the escalation could leave “children and families trapped between fighting lines or caught in the crossfire, with fatal consequences.”

She said that families had already experienced “harrowing journeys” to reach safety early in the war. “Now, even that fragile sense of security is shattered as those same children have once again been forced from their homes,” she added.

More than 7 million people were forced out of their homes, including more than 1.5 million who have sought refuge in neighboring countries, according to the U.N. figures. Chad received more than 500,000 refugees, mostly from Sudan's western region of Darfur, where the RSF conquered much of its areas.

The fighting in Wad Medani forced many aid groups, including the International Committee of the Red Cross, to evacuate its staff from the city, which was a center of the humanitarian operations in the country.

The RSF takeover prompted fears among Wad Medani residents that they would carry out atrocities in their city as they did in the capital, Khartoum, and Darfur. The U.N. and rights groups have accused the RSF of atrocities in Darfur, which was the scene of a genocidal campaign in the early 2000s.

The RSF grew out of the state-backed Arab militias known as Janjaweed, which were accused of widespread killings, rapes and other atrocities in the Darfur conflict.

Ahmed Tag el-Sir, a father of three, fled along with his family to the neighboring province of al-Qadarif after the RSF rampaged through their village of al-Sharfa Barakar north of Wad Medani.

“They shelled the village and took over residents’ homes, like they did in Darfur,” the man said from a relative’s house where he shelters along with two other families. “We fled out of fear of being killed or our women being raped by the Janjaweed.”

Fighting reaches Sudan’s Wad Madani, once considered a safe haven for displaced.

Thousands of people have been forced to flee as fighting reached one of Sudan's largest cities, Wad Madani.

General Mohamed Hamdan Dagalo, commander of Sudan's Rapid Support Forces (RSF) paramilitary group, said on social media that the RSF have taken control of Wad Madani, capital of Sudan's el-Gezira state; videos posted on social media appeared to show fighters driving along the city's streets, singing in celebration.

Fighting first erupted in Sudan on April 15, a culmination of weeks of tensions linked to a planned transition to civilian rule. General Abdel-Fattah Burhan, commander of the Sudanese Armed Forces (SAF) and General Mohammed Hamdan Dagalo head of the Rapid Support Forces -- once allies who jointly orchestrated a military coup in 2021 -- are engaged in a vicious power struggle.

The RSF's advance into Wad Madani comes following days of heavy fighting -- including airstrikes and shootings -- between the RSF and the SAF. The Armed Forces announced on Tuesday its soldiers have withdrawn from their positions in the city.

Local authorities have announced a state of emergency, imposing a curfew in Wad Madani effective from Dec. 15 until further notice.

As fighting rages thousands have been forced to flee the city which was considered an oasis and "safe haven" in central Sudan, hosting thousands of persons, including thousands from Sudan's capital Khartoum, who had been earlier displaced by the war.

"I am a Khartoum resident, and we had to flee in April to Wad Madani," Omnia Elgunaid told ABC News over the phone. "And now I had to flee Wad Madani again to a nearby village."

"My friends are scattered, some of them I cannot reach them, and I have no idea what happened to them if they're okay, if they're alive or not," Elgunaid added.

Elgunaid is just one of the estimated 14,000 to 15,000 civilians who have been newly displaced from the Madani locality, according to the International Office for Migration's (IOM) Displacement Tracking Matrix.

Along with serving as safe area for thousands displaced civilians, Wad Madani was a crucial humanitarian aid hub, causing fears humanitarian operations may be in jeopardy.

World Health Organisation (WHO) Chief Tedros Ghebreyesus said the WHO is "extremely concerned" at the unfolding situation, announcing the WHO's operations hub is temporarily suspended.

"We're witnessing one of the largest displacements in the world… the health system is at a breaking point, while the needs are sharply rising," he said.

In a statement sent to ABC News, Doctors Without Borders said hospitals have been receiving wounded people since Friday: "We are particularly concerned about the impact of the intensified violence on the people who live here, as well as the danger of repeated displacement for those who have already fled from other areas."

After about 8 months of fighting, at least 12,000 people have been killed, according to the UN figures. At least 6.7 million people have been displaced within and outside Sudan's borders since fighting broke out in April 15.

According to UN Relief Chief Martin Griffiths, over half a year of war has plunged Sudan into "one of the worst humanitarian nightmares in recent history."

In a statement sent to ABC News, a U.S. State Department spokesperson said it is "deeply concerned" by reports of the RSF's advance in Gezira state and against Wad Madani.

"The current RSF offensive threatens vulnerable civilians and disrupts humanitarian assistance efforts," the spokesperson said. "We urge the RSF to cease these actions, which are inconsistent with its stated claims that it is fighting to protect Sudan's people."

They added, "We urge SAF General Burhan and RSF General Hemedti to hold those responsible for attacks on civilians to account, and to abide by their IGAD summit commitments of an unconditional ceasefire and a face-to-face meeting between them."