Sudan has ejected 20 United Nations staff working in the Darfur region, the UN refugee agency UNHCR said on Tuesday.
Of UNHCR's 37 international staff in Darfur, 20 have not had their work permits renewed, UNHCR spokeswoman Melissa Fleming told a regular UN briefing in Geneva.
"Most of them were asked to leave at short notice in July. As a result we've been unable to effectively undertake our work there," she said.
UNHCR has not had any explanation for the expulsion of its staff members, all of whom were based at El Fasher in north Darfur, which Fleming described as "a major operational hub".
She did not reveal details about them but said their expulsion did not appear to be related to their nationalities.
The decision not to renew the permits was thought to have been taken by the government in Khartoum, not by local officials, Fleming said.
Other UN agencies, such as UNICEF and the World Food Programme, have not had any problem with renewing work permits.
Violence is down from its peak in 2004-05, but has picked up again this year as Arab tribes, many of which were armed by government early in the conflict, are now fighting among themselves over resources and land.
Fleming said the humanitarian situation was "critical, unstable, volatile, both for the people who have been displaced for years on end but also for many newly displaced people".
In April, about 50,000 Sudanese fled into Chad after a bout of conflict in Darfur. But the displacement into Chad has "pretty much stopped" very recently, Fleming said.