(Bloomberg) -- Sudan’s paramilitary Rapid Support Forces defied a US warning and expanded its offensive on a crucial aid hub, escalating the eight-month-old conflict and forcing hundreds of thousands of people to flee.

The four-day attack on the southeastern city of Wad Madani jeopardized the North African nation’s main humanitarian operations, already running at low capacity in a war between the RSF and Sudan’s military that’s left 12,000 people dead and displaced millions more.

As many as 300,000 people have fled the area in Sudan’s Al-Jazirah state — located on the Nile River and commonly referred to as Sudan’s breadbasket — since Dec. 15, the United Nations said Tuesday in a statement. “Given the scarcity of available transport options, many people have reportedly fled by foot,” it said.

Sudan’s military said in a statement it had withdrawn its forces from the city of Wad Madani and that an investigation had been launched to understand the reasons for the collapse of the army’s forces.

The escalation has thwarted efforts by East African mediators and regional heads of state as well as the US and Saudi Arabia to have the leaders of Sudan’s two warring factions meet face to face for the first time in the conflict by the end of the year. The UN is warning of a humanitarian catastrophe, with 25 million — more than half of the resource-rich country’s population — in need of aid.

Read More: What’s Behind the Fighting in Sudan and What It Means: QuickTake

Violence also spread in the southwestern city of El-Fasher in the Darfur region, where hundreds of people fled and private and commercial properties were looted in recent days. The upheaval has spilled over into Sudan’s eastern state of Sennar, with people trying to reach the relative safety of Gedaref, Kassala and Blue Nile states.

The attack on Wad Madani has shuttered shops and markets. It’s also put at risk warehouses containing large stocks of supplies for displaced people and residents of the capital, Khartoum.

All humanitarian field missions have been canceled since Dec. 15, according to the UN.

The RSF, which has its roots in a Darfur militia known as the janjaweed, or “devils on horseback,” said in a statement it moved on Wad Madani after receiving information the army under Abdel Fattah al-Burhan mobilized 40,000 soldiers in an effort to eliminate its forces. Wad Madani is located 150 kilometers (93 miles) southeast of Khartoum.

A spokesman for the army didn’t respond to questions on the latest military operations.

The spike in violence occurred around the fifth anniversary of a popular revolt that eventually toppled the former dictator Omar al-Bashir, whose whereabouts remain unknown after he left a prison in Khartoum earlier in the conflict.

“The incredible bravery and sacrifice displayed by protesters from all walks of life – especially women and youth – led to the beginnings of a democratic future,” the US Embassy to Sudan said in a statement. “However, the promise of that future has been denied to Sudan’s people time and again.”

Cameron Hudson, a senior associate at the Center for Strategic and International Studies Africa Program, said he was concerned about who might support the army now that it was under pressure.

“I think we should not assume that the army is just on a gliding path toward defeat,” he said, noting that it could get further support from other nations.

(Updates with comment from the army in fourth paragraph and analyst quotes.)

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