UN Secretary General Kofi Annan has threatened to suspend aid to Darfur if the "completely unacceptable" kidnapping of African Union monitors by rebels continues.
The message came after an AU spokesman confirmed that Darfur rebels had released 36 African Union monitors kidnapped on Sunday, but were still holding two more.
The AU had blamed a breakaway faction of the Justice and Equality Movement (JEM) for the kidnappings, an accusation denied by the faction.
Speaking in Geneva, Annan said: "Both rebels and government must understand that, if these incidents continue, it will impede humanitarian assistance and delivery.
"It's already impeding access to some of the people in need and it may require a cessation of operations in some parts of the territory," he added.
Annan added that it was Sudan's duty to protect both aid workers and AU peacekeepers on its territory, even if it was rebels who were responsible for the hostage-taking.
