(Adds quotes from U.N. memorandum, background)
By John Kanyunyu
GOMA, Congo, Feb 25 Rwandan troops began withdrawing from Congo on Wednesday, stoking fears that Rwandan Hutu rebels will step up reprisals against civilians and retake ground they lost during a month-long offensive against them.
Rwanda sent thousands of troops into Congo's North Kivu province last month and the former enemies have been conducting joint operations against Rwandan Hutu rebels seen as a root cause of 15 years of conflict in the region.
Officials have said all Rwandan troops are due to leave in the next few days, following intense pressure on Congo's President Joseph Kabila from domestic critics who note that previous Rwandan forays into Congo during two recent wars were marked by alleged abuse and looting of natural resources.
Hundreds of soldiers crossed the border in the eastern Congolese town of Goma, most marching on foot, others riding on military trucks, to be greeted by cheering crowds in Rwanda.
The pullout has prompted fears within the United Nations' biggest peacekeeping force that the mainly Hutu Democratic Forces for the Liberation of Rwanda (FDLR) rebels, who have mostly avoided clashes with the joint force, will seek revenge.
"The planned pullout of (Rwandan) troops raises concerns about the protection of the civilian population, given the limited capacity and professionalism of (Congolese) troops," read an internal U.N. memorandum, which was seen by Reuters.
"Although the FDLR has mostly vacated the areas upon the arrival of the joint forces, it is likely that they are waiting in the bush for the forces to retreat and then come back to retaliate on civilians perceived as being traitors," it said.
"In this context, incidents of FDLR attacking civilian populations, looting raping and killing, are on the rise."
The U.N. is investigating at least 15 killings and nearly 90 kidnappings between Feb. 15 and Feb. 21 against villagers accused of collaborating with the joint force, said the report, addressed to Alan Doss, head of the U.N. mission (MONUC).
New York-based Human Rights Watch said earlier this month that the rebels had massacred more than 100 civilians accused of betraying them since the start of the operations on Jan. 20.
Doss's stretched mission has been accused of failing to do enough to prevent attacks on civilians during the operations.
Congo and Rwanda, who have long blamed the U.N. for failing to tackle the rebels as well, declared the operation a success.
The Congolese commander of the offensive, General John Numbi, said late on Tuesday that Rwandan and Congolese soldiers had killed 89 FDLR fighters and forced 140 more to surrender.
"The joint operations in North Kivu ... have demonstrated to the entire world that Africans can do many things without necessarily waiting for outside help," Numbi's Rwandan deputy for the operations, General Jerome Ngendahimana, said in Goma.
Congo has vowed to broaden the drive against the estimated 6,000-strong rebel force by taking operations in South Kivu province with MONUC support "until the threat is eliminated".
However, the MONUC document warned that, without the Rwandan presence, U.N. soldiers would be hard pressed protect civilians.
The U.N. refugee agency UNHCR said on Tuesday that FDLR attacks were sparking a new wave of displacements.
MONUC has already been criticised by aid groups and rights campaigners who say the mission has not done enough to prevent the reprisal killing of nearly 1,000 villagers by Ugandan rebels targeted by a similar joint army effort in northern Congo. (Additional reporting and writing by Joe Bavier; Editing by Louise Ireland and David Lewis)