"There are too many shifting allegiances at the moment," said Gillian Wilcox, spokeswoman for Operation Lifeline Sudan, an umbrella group of U.N. agencies and aid organisations working in Sudan.
"Until we get a clearer idea of who's in control we have to suspend our flights," she told Reuters.
A pro-government militia seized a U.N. plane and the four people on board in the southern region last Thursday, and Wilcox said the United Nations was still trying to negotiate their release.
The four, an American U.N. worker, two Kenyan pilots and a Sudanese relief worker, were taken hostage when they flew into the Upper Nile town of Old Fangak.
Aid agencies fly to southern Sudan daily to deliver relief supplies to a region which has been devastated by a 17-year-old civil war and lurches from one hunger crisis to the next.
In its broadest terms, the war pits the Islamic government in Khartoum against mainly Christian and animist rebels of the Sudan People's Liberation Army in the south.
But the picture is complicated by inter-factional fighting, especially in oil-rich Upper Nile where numerous pro-government and rebel militia leaders are fighting for supremacy.
The U.N. and aid agencies have been unable to reach many areas of the region for several months because of the lack of security and because the government has banned flights to some locations.
- Reuters - Thomson Reuters Foundation
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