The BBC's Andrew Clark: "Another blow to the UN mission in Angola"

The move comes after two UN aircraft were apparently shot down in the past week.

UN Special Envoy Issa Diallo said it appeared the UN was being targetted.

He did not rule out a complete UN pull-out from Angola, although he made it clear he would be reluctant to take such a step.

Earlier in New York, the UN Secretary-General, Kofi Annan, ordered the immediate suspension of its flights around the city of Huambo.

Mr Annan said he was outraged by the attacks and called on the government and rebels to halt their fighting.

No word on survivors

Lara Pawson in Luanda: "The UN is thinking seriously about its future role in Angola"

UN observers say the latest plane to crash had eight people on board - four UN personnel and four crew members. It is not known if there are any survivors.

The plane shot down shortly after take-off from the city of Huambo in Angola's central region.

The UN spokesman in Luanda, Hamadoun Toure, told the BBC that conditions for their personnel in Angola had become very difficult.

UN spokesman Hamadoun Toure: We're here to monitor peace not war

Just a week ago, a UN cargo aircraft carrying 14 people crashed less than 50km from the scene of the latest crash. Ten UN officials and four crew were on board.

The Australian Government has expressed concern at conflicting reports on the fate of the aircraft passengers - which included at least one Australian - and has called for unhindered access to the crash sites.

Unita has denied involvement in the crash and has dismissed government claims that the rebel movement was holding survivors.

UN officials have been trying to contact the Unita leadership to establish the exact situation.

The Angolan Government issued security guarantees to allow a UN team to travel to the site.