ROME -- Up to 2 million more refugees could flee Rwanda if UN peacekeeping troops are not ready to take over when French forces withdraw in two weeks' time, a senior United Nations official said on Tuesday.


The UN under-secretary general for humanitarian affairs, Peter Hansen, called on UN members to be ready to take over when 1,200 French soldiers withdraw from the southwestern area of the central African state on Aug. 22 when their two-month mandate expires.


"If the international community is not ready to put its actions where its words are and prevent a breakdown of the situation in the southwest, it would create a vacuum that would lead to instability," he said.


"We could very well see an outflow of between 1 and 2 million displaced people going across the border into Bukavu in Zaire, he told reporters in Rome.


Nearly 1 million refugees fled across the border to Goma in eastern Zaire three weeks ago. Only a trickle have returned since the victory of the mainly Tutsi Rwanda Patriotic Front after three months of civil war.


Hundreds of thousands of Hutus, whose kinsmen have been widely blamed for the massacres of 500,000 Tutsis and Hutu opponents of the ousted government, are sheltering in French-patrolled safe havens in the southwest. Many have threatened to leave if the French go, with the Zairean border town of Bukavu, which is already overwhelmed by 300,000 refugees, their most likely destination.


So far, the UN is still some 1,500 troops short of the 5,500 it required to keep the peace in Rwanda, where the civil war erupted four months ago.


The UN has asked France to stay on after Aug. 22, fearing that its planned withdrawal from the safe zone would trigger renewed bloodshed between Hutus and Tutsis.


France has not ruled out keeping its troops in the zone beyond the expiry of its mandate but says it plans to stick by the deadline for UN troops to take over.


An estimated 500 soldiers from Chad, Congo, Guinea-Bissau, Niger and Senegal, are already in the area and are expected to join the UN Assistance Mission in Rwanda set to replace the French troops.


The Rwandan massacre sparked one of the world's largest single movements of refugees. The price ticket for relief had soared to around $430 million as a result of the exodus, Hansen said.


That was way beyond the $270 million worth of aid that had been estimated only one month ago before the exodus.


"It is frightening to see how fast a moving target we are shooting at," Hansen said.


Donor nations had already pledged about half the $430 million and Hansen said he was confident that the rest would be received by the end of the year.


Aid officials fear lack of food in the southwest, which was gripped by famine before the war, may also make refugees leave.


"We know the relief situation in the southwest is very bad. There are simply not enough supplies," Chris Janowsky, Kigali spokesman for the UN high commissioner for refugees, said. Janowsky added that the UN was opening an office in Gikongoro in the southwest.


The World Food Program, the UN food arm, delivered 340 tons of food to Gikongoro on Monday and said it planned three or four convoys each week to meet refugee needs. The program has been struggling to cope with the competing needs of the different camps in the various areas. It has found warehouses in Bukavu but says it cannot stockpile food before another disaster as it has no money and can hardly meet the 50 tons a day needed to feed the 300,000 already there.


In Kigali, a U.S. military spokesman said a total of 280 U.S. troops were based at the capital's airport helping deliver aid but the number was expected to drop within days to some 200.


"I believe we are at the maximum now you will see for U.S. forces and the plan is now to gracefully degrade those forces while we transfer those services to contractors," U.S. air-force Lieutenant Colonel Randal Morger told reporters.