UN Secretary-General Kofi Annan asked the Angolan government and Unita rebels for help in finding the crash site, which is thought to be near Vila Nova, 25 km (16 miles) from the city of Huambo.

It is not known if there are any surviviors from the 10 passengers and four crew who set out on UN Flight 806 from Huambo to Saurimo.

Annan - "great sadness" over crash

"The secretary-general appeals to the Angolan government and Unita to extend all possible assistance to enable the UN Observer Mission in Angola (Monua) to reach the crash site and to assist in search-and-rescue operations for any survivors," the statement said.

In Lisbon, the Portuguese news agency, Lusa, quoted the UN mission in Angola as saying search parties were not expected to leave Huambo before Sunday morning because of continued fighting in the area.

It is not known if those on board were UN observers who have been trying to help implement the 1994 Lusaka agreement between Unita and the government.

The UN operation in Angola has suffered setbacks from air crashes before. In June, UN envoy Alioune Blondin Beye died when his plane dived into a mangrove swamp in the Ivory Coast.

Peace accord collapses

The highland provinces of Huambo and Bie have been at the centre of fierce clashes between rebels and troops loyal to the government of President Jose Eduardo dos Santos 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.

In the latest clashes, the strategically-important town of Kuito again came under heavy artillery fire from Unita rebels.

Peter Biles reports on the collapse of the Lusaka accord

The Portuguese news agency Lusa says 31 people - many of them women and children - were killed and 36 wounded on Friday.

Angolan government military forces said nine civilians were killed when mortar fire hit a Roman Catholic church in the suburb of Cangoti.

Earlier this week aid workers in Angola warned of a looming humanitarian disaster in the country as a result of the fighting in Bie province.

A spokeswoman for the Red Cross said Kuito faced an influx of refugees and wounded people and there was no means of bringing in medicine or food.

The fighting around Kuito has destroyed the harvest and left up to 100,000 refugees on the brink of starvation.