Refugees have taken 34 aid workers hostage in Darfur's largest refugee camp and demanded the release of one of their leaders arrested by police, UN officials said.
It is not clear if any Irish citizens are among the kidnapped workers.
"We are aware that some NGO (non-governmental organisation) workers have been taken hostage within Kalma camp," a UN spokesman said.
Another source said negotiations were in progress on the release of the refugees' tribal leader in the hope that this would secure the release of the hostages.
Earlier this month, a United Nations envoy said Arab militias had escalated their campaign of violence against civilians in Darfur, Sudan, mounting a pair of attacks against camps for displaced families.
Juan Mendez, special adviser to UN secretary general Kofi Annan, said Khartoum had not abided by a long-standing Security Council order to establish a "plan to disarm" the Sudanese-backed Arab militiamen, who are accused of driving more than two million tribal Africans from their homes since 2003.