The United Nations has ordered the withdrawal of about 1,000 peace monitors from battle zones in Angola and is deliberating the future of its entire mission after Unita rebels shot down a second UN aircraft within a week.
The Hercules transport aircraft with eight people on board, including four foreigners, was hit by anti-aircraft fire after flying out of the besieged central city of Huambo on Saturday.
The UN has called a halt to all its flights in Angola while it tries to discover if there are any survivors and considers whether it can play any useful role as fighting between government forces and Unita worsens.
The UN Secretary-General, Mr Kofi Annan, called for an immediate ceasefire to allow search teams to look for survivors from the latest crash and the shooting down a week earlier of another Hercules over rebel-held territory near Huambo.
Unita denies government accusations that it is holding the 14 passengers and crew on board the first plane.
The UN chief in Angola, Mr Issa Diallo, noted that other aircraft flew from Huambo on Saturday without coming under attack and that both sides were informed beforehand of the downed aircraft's flight plan. They included four government planes and two from the World Food Programme.
"We can't say that because there is a war, we're leaving and that's it. We can't abandon this population," he told AFP.
According to Mr Diallo, the United Nations is studying an option under which what he described as UN "combat units" accompanying aid convoys would be ordered to fire on attackers to ensure that aid reaches the people.
He blamed Unita for the crisis because it had failed to disarm many of its fighters in line with the 1994 accords signed in Lusaka, Zambia. He said counterattacks arose from "the frustration of the government". The UN is viewed with contempt by both sides in Angola. The government began to lose faith before the 1992 presidential elections when the UN mission overseeing the first round of peace accords glossed over Unita's failure to disarm and demobilise. The then head of the UN delegation, Ms Margaret Anstee, argued that the problem could be resolved after the vote.
However, when Dr Jonas Savimbi, the Unita leader, lost the election he accused the UN of helping to rig the ballot and began re-deploying his forces.
Senior Angolan government officials accused Ms Anstee of repeatedly making concessions to Dr Savimbi in the hope of rescuing the election, but all the UN achieved was to buy time for Unita to occupy large tracts of rural Angola.
Ms Anstee's replacement, Mr Alioune Blondin Beye, had a style of caressing and cajoling that helped secure a new peace accord in 1994 with promises of power-sharing, but Dr Savimbi, who became vice-president, could not bring himself to serve under a government he had spent nearly two decades fighting, and repeatedly failed to make it to Luanda to take up his new position.
After Mr Blondin Beye was killed in a plane crash in June, the peace accords again unravelled. Dr Savimbi refused to hand over rebel-held territory to government control and began reoccupying territory. In recent days the UN Security Council has taken both the Angolan government and Unita to task. It gave the government until January 11th to co-operate with the search for survivors from the first downed plane. But the deadline is more than a fortnight after the aircraft was shot down behind rebel lines.