The downing on Saturday of the C-130 cargo aircraft with eight people on board is the latest indication that a peace accord brokered by the UN in 1994 between Unita and the government is rapidly unravelling.
In New York, Mr Annan said: "All threats to UN personnel - in the air and on the ground - must cease immediately." In a statement, he called on the Angolan government and Unita to declare "an immediate ceasefire which will permit the conduct of search-and-rescue missions, as well as the relocation of United Nations staff to safer areas".
The aircraft was part of an evacuation effort to withdraw UN personnel and equipment from the central city of Huambo.
About 100 UN staff remain in the city. It is in an area held by Unita (National Union for the Total Liberation of Angola), where fighting has recently increased. The plane was hit by an anti-aircraft missile 20 minutes after take-off. It was carrying four Angolans, two Filipinos, a Namibian and an American.
On 26 December, another UN-chartered cargo plane with 14 people on board, including eight UN peace-keepers, was shot down in the same area.
Angola state radio has since reported that survivors of the earlier crash are being held by Unita rebels. The rebels deny the claim.
Last Thursday, the UN Security Council held an emergency meeting to discuss developments in Angola. The council decried the shooting down of the first aircraft and condemned "the incomprehensible lack of co-operation clarifying the circumstances of this tragedy".
The council asked Mr Annan to decide by 15 January whether all UN peace- keepers should be withdrawn from the country. The UN has some 1,000 observers in Angola, but only a few of these are in areas where fighting is currently under way.
Mr Annan said yesterday: "There can be no excuse for further delay on any side in disclosing all relevant information about the fate of the passengers and crews of the two UN aircraft".
Issa Diallo, head of the UN Observer Mission in Angola, defended the decision to proceed with the evacuation. He pointed out that other aircraft, including two chartered by the World Food Programme, had departed from Huambo on Saturday without incident. This suggested UN-chartered aircraft were being singled out. "The UN plane asked and got authorisation, takes off and gets shot down," he said.
The UN-brokered peace accord of 1994 was designed to end almost two decades of fighting in Angola. Fears were growing yesterday that the oil- and-diamond-rich southern African nation is about to resumes the civil war which started with independence from Portugal in 1975.Reuse content