Mr Annan called for an immediate ceasefire by the Angolan Government and Unita rebel forces. This would allow search-and-rescue missions and the relocation of UN personnel to safe areas.

UN observers say the plane had eight people on board - four UN personnel and four crew members.

It was shot down shortly after take-off from the city of Huambo in Angola's central region. It is not yet known if there are any survivors.

Meanwhile, the UN has temporarily suspended all air operations in the Huambo region.

Lara Pawson in Luanda: Uncanny similarities between the two crashes

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

The BBC's correspondent in Luanda, Lara Pawson, says the latest incident could see the UN pulling out many of its workers from the most dangerous parts of the country, prompting fears of a humanitarian crisis.

Passenger mystery

The BBC's Anna Richardson reports on Angola: The Forgotten Vietnam

Just a week ago, a UN cargo aircraft carrying 14 people less than 50km away 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.

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

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

Unita's Secretary for Foreign Affairs, Alcides Sakala, told the BBC that claims by the Angolan army that survivors from the crash were being held by the group were "absolutely false" and "cheap propaganda".

Mr Sakala said it was unlikely there were any survivors from the crash because the plane had exploded in mid-air. However he said attacks by government fighter aircraft had prevented him from receiving information from the front line.

"There can be no excuse for any delay on any side in disclosing all relevant information about the fate of the passengers and crews of the two UN aircraft," Kofi Annan said.

Security guarantees sought

Thousands of refugees have flooded into Huambo

On Friday, Brigadier Jose Manuel Jota of the Angolan army said an unknown number of passengers had survived last weekend's crash and were being held near the crash site in the Unita-controlled areas of Bailundo and Andulo.

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

Now the government has issued security guarantees to allow a UN team to travel to the site - 500km south-east of the capital, Luanda - and the UN says it is seeking similar guarantees of safety from Unita.

Huambo, Angola's second largest city, is held by the government and is used by the army to fly in reinforcements for fighting in surrounding areas.