Bodies have lined the streets in Sudan's western Darfur region as the UN said Tuesday that more than two months of fighting have forced over 2.5 million people from their homes.

A three-day ceasefire due to end Wednesday at dawn brought a brief respite to the capital Khartoum, gripped by the war that erupted on April 15 between two rival generals.

The fighting has killed more than 2,000 people nationwide, the Armed Conflict Location and Event Data Project said.

According to the United States State Department, up to 1,100 have been killed in the city of El Geneina alone, the capital of West Darfur state in a region wracked by some of the bloodiest violence.

The UN has spoken of possible "crimes against humanity" in Darfur, where the conflict has "taken an ethnic dimension", the world body said in a statement with the African Union and east African regional bloc IGAD.

Bodies have remained on the streets of El Geneina, where months of unrest have left shops either vacant or gutted by looters.

One lay covered on the asphalt, in front of an armoured vehicle. A dead man was partially curled up outside a house. Several others appeared to be lying face down together on a dirt road.

Residents have fled the city en masse, many grabbing whatever they could to flee to the border with Chad.

Some described being shot at by fighters and subject to searches during the perilous journey.

The conflict sees the army, led by Abdel Fattah al-Burhan, battle the paramilitary Rapid Support Forces (RSF) commanded by his former deputy Mohamed Hamdan Daglo.

State Department spokesman Matthew Miller has attributed the West Darfur "atrocities" primarily to the RSF, and the UN's Sudan mission chief, Volker Perthes, referred to reports of attacks "allegedly committed by Arab militias and some armed men" in RSF uniform.

In a social media video Tuesday, Daglo denounced "a tribal conflict" in El Geneina.

"Almost 900 wounded and 15,000 Sudanese refugees from West Darfur's capital and its surroundings have reached the Chadian town of Adre in the last four days," the Doctors Without Borders (MSF) charity said on Monday.

At least 150,000 people have fled Darfur into Chad since the start of fighting, according to the UN.

- 'Lawlessness' -

The number of people uprooted from their homes by the conflict has topped 2.5 million, including about 550,000 who have fled abroad, according to the International Organization for Migration.

Filippo Grandi, the head of the UN's refugee agency, on Tuesday urged Sudan's neighbours to keep their borders open despite security worries.

"My appeal to all the neighbouring countries is to say I understand your security concerns, but please keep your borders open because these people are really fleeing for their lives," he told AFP in an interview.

Donors gathered Monday at a conference in Geneva where they pledged almost $1.5 billion -- only half of the estimated needs to combat the humanitarian crisis in Sudan and help its neighbours host those fleeing the fighting.

More than half of Sudan's population, 25 million, require aid, the UN says.

"The humanitarian needs have reached record levels in Sudan and there is still no sign of an end to the conflict," said Eddie Rowe, the World Food Programme's Sudan director.

Alexander Kjaerum of the Danish Refugee Council described as "absolutely shameful" the funding levels for the conflict, comparing it to the outpouring of support received at the start of the war in Ukraine.

The latest in a series of truces that have all been systematically violated began Sunday and is due to end at 6:00 am (0400 GMT) Wednesday.

Although it brought a relative respite, the International Committee of the Red Cross (ICRC) on Monday said it was "not respected".

An operation it carried out to transfer wounded soldiers to a hospital "had to be aborted after gunshots sounded in the immediate proximity of our convoy", the agency said.

UN chief Antonio Guterres on Monday warned that "the scale and speed of Sudan's descent into death and destruction is unprecedented".

"Without strong international support, Sudan could quickly become a locus of lawlessness, radiating insecurity across the region," he said.

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