United Nations food convoys to Sudan's western Darfur region have been suspended because of fighting.
The deteriorating security situation is hampering aid efforts
The World Food Programme stopped its deliveries on Monday when rebels attacked police stations in the neighbouring state of West Kordofan.
Road closures prevented 70 trucks carrying rations for some 260,000 people reaching Darfur.
Despite an April ceasefire, clashes between rebels, pro-government militias and the army continue.
The UN has called Darfur one of the world's worst humanitarian crises, with some two million refugees relying on aid handouts since the conflict began nearly two years ago.
Security on the ground continues to deteriorate and the African Union troops sent to Darfur to protect the ceasefire monitors are having little impact on the fighting.
"We're talking about a major humanitarian delivery that was stopped," the UN's Radhia Achouri in the capital Khartoum told the BBC's Focus on Africa programme.
"We have decided to not proceed until further notice, until the security situation is reassessed," she said.
Both Darfur rebel groups - the Sudan Liberation Army and the Justice and Equality Movement - have denied responsibility for the attacks in Ghubaysh, she said.
The situation was confusing, she added, as reports suggest a new rebel movement is claiming responsibility for the attack on the main road to the towns of Nyala in south Darfur and El-Fashir in the north.
In another development, the UN is also concerned about the theft by rebels of 13 trucks carrying food aid over the past two weeks.
Ms Achouri said there were worrying reports that rebels were using the WFP lorries to launch their attacks.
Jan Pronk - the UN's special representative to Sudan - called for the immediate return of the trucks.
"The problems of Darfur cannot be solved through military means," Mr Pronk said in a statement.
Faltering peace talks between the government and rebels have been adjourned until January.
Last week UK charity Save the Children announced it was pulling out of Darfur after some its staff were killed in attacks.