Escalating violence in Sudan's Darfur region could lead to the United Nations halting relief efforts there, says their chief aid coordinator.
More than 2m have fled their homes in Darfur
Jan Egeland said violence in Darfur had become so bad that the UN operation could "all end tomorrow - it's as serious as that".
He was speaking as reports were made that 29 refugees were killed by Arab militias in their refugee camp.
Mr Egeland said international aid workers were being targeted.
The UN refugee agency said it had received reports that some 300 armed men attacked the Aro Sharow camp in West Darfur.
They burnt 80 shelters, forcing thousands of people who had already fled their homes to run away once more.
Violence has also flared on the border between Darfur and Chad.
At least 36 people were killed in eastern Chad after gunmen on horseback - said to have crossed the border from Darfur - raided the village of Madayouna in an apparent attempt to steal cattle.
"Armed horsemen wearing military uniforms and coming from Sudan penetrated [on Monday] into Chadian territory," the Chadian government said in a statement.
The military killed eight and captured seven of the attackers, the government added.
On Tuesday, the UN criticised the Sudanese government over its peace efforts.
Special UN advisor on preventing genocide, Juan Mendez, said Khartoum had done little to disarm militias or end the "culture of impunity" there.
He said Sudan's authorities remained in denial about the extent of the problem.
More than two million people have been forced from their homes and at least 180,000 have died during conflict between the government and rebel groups in Darfur.