A WFP spokesman said the food delivery flights were being called off until Wednesday as a precautionary measure until more was known about the crash.

The WFP flights are a lifeline for thousands of refugees who have fled to the provincial capital of Huambo, to escape weeks of heavy fighting in central Angola.

The Angolan authorities have accused Unita rebels of shooting down the plane, but there has been no independent confirmation of the cause of the crash. Unita in turn has said it will investigate the crash.

The UN observer mission said the plane burst into flames after taking off from Huambo, which has been the target of Unita shelling in recent weeks.

Attempts to locate the wreckage of the plane are being hampered by continuing fighting.

The United Nations has called for a 48-hour ceasefire between government forces and Unita to allow the search for the wreckage, but UN spokesman Hamadoun Toure said in Luanda that neither side had yet responded to the appeal.

Even if fighting does cease, the search for the wreckage is likely to be hampered by torrential rain and the presence of landmines in the area.

Fears for refugees

Thousands of refugees depend on the aid flights for food

UN World Food Programme spokeswoman Brenda Barton said the agency had been flying into Huambo at least twice daily to replenish food stocks in the city, which has become a temporary home to an estimated 100,000 people fleeing the conflict elsewhere in the region.

She said the refugees would be the worst affected by the decision to suspend flights.

"We had hoped to bring in 350 tons over a two-week period, obviously we have been interrupted not even half way," Ms Barton said.

"The situation is very grave. Our people on the ground say there's a steady flow of people arriving in the town with severe malnutrition."

UN observers killed

The passengers aboard the plane included three Angolans, two Russians, an Australian, one Egyptian, one Cameroonian, a Zambian and a Namibian, Mr Toure said.

Eight were members of the UN Observer Mission in Angola, and two were employed by a private communications company which was operating with the UN.

The crew included a South African, an Angolan, a Bolivian and a Filipino.

Weeks of fighting

The highland provinces of Huambo and Bie have been at the centre of fierce clashes between Unita and government forces since early this month.

The fighting marked the final collapse of a precarious four-year-long peace accord intended to put an end to two decades of brutal civil war.