The UN peacekeeping mission in Darfur remains understrength

In the latest incident, seven trucks were stolen and their drivers abducted last week as they drove to Fasher, the World Food Programme says.

WFP says 37 trucks and 23 drivers are still missing and other drivers are unwilling to risk going to Darfur.

Some two million people rely on food aid as a result of the conflict.

The WFP also warns it could halt its Humanitarian Air Service which transports aid workers around the vast country at the end of this month because of a lack of funding.

"This is an unprecedented situation," said WFP representative in Sudan Kenro Oshidari.

"Our humanitarian air operation for aid workers could be forced to stop flying because we have no money, at a time when our helicopters and aircraft are needed more than ever because of high insecurity on the roads."

Inaccessible

WFP does not say who is behind the attacks in Darfur - there are numerous rebel groups and pro-government militia in the region.

Some 8,000 people a month use WFP flights in Darfur - 3,000 in helicopters to reach parts of the country which are otherwise inaccessible, the UN agency says.

Aid agency Oxfam has warned that its operation in Darfur would be at "serious risk" if the WFP flights stopped or were reduced.

WFP also operates flights around South Sudan, which is slowly recovering from its own long conflict and where infrastructure is even worse than in Darfur.

Here too, a massive aid operation to help millions of people affected by the war would be badly affected if flights were stopped.

The UN's special rapporteur for Sudan Sima Samar accuses says civilians were used as human shields in recent clashes in West Darfur.

Fighting has increased recently there, leading to a new stream of refugees.

The joint UN-African Union peacekeeping force is making little progress - it still has just 9,000 troops out of the 26,000 planned.