The FDLR are hiding in the picturesque mountains around Pinga
A Hutu rebel group in Democratic Republic of Congo is said to have drawn up a "hit list" of those who helped a military operation against it.
A UN official in eastern DR Congo told the BBC that the joint attack this year by the Congolese and Rwandan armies had failed to neutralise the rebels.
Capt Karl Fuller said 800 fighters had tried to attack the village of Pinga in eastern DR Congo on several occasions.
FDLR leaders are accused of taking part in the 1994 genocide of Rwandan Tutsis.
After years of mutual animosity between DR Congo and Rwanda, the two armies worked together in an operation against the FDLR (Democratic Liberation Forces of Rwanda) in January.
But Capt Fuller said the five-week joint operation had only succeeded in scattering the rebels.
"They've been here for the past 10 years, so they know this area, it's known to them and they have fled their specific strongholds to the mountainous areas and they were not found," he told the BBC. "They did not manage to neutralise them at all."
The South African added: "There's [civilian] elements here that have physically told the RDF [Rwanda Defence Force] when they came in: 'This is where the FDLR strongholds are, go there and take them out.'
"But now they've come to hear of it, the FDLR has even put out a hit list."
He said the UN peacekeeping force, known as Monuc, had fought off several attacks by hundreds of FDLR militiamen on Pinga.
The village, which lies more than 100km (60 miles) north-west of Goma, the capital of North Kivu province, is particularly vulnerable as it is surrounded by mountains, he added.
One villager told the BBC: "When the Rwandan army was here, the FDLR fled, but when the Rwandans left, the FDLR came back. We feel unsafe again since they are still in the forest."
The BBC's Thomas Fessy in Pinga says there are also plenty of mines around the village, many of them run by the FDLR.
He says is is protected by 170 UN peacekeepers but only 70 troops from the Congolese army.
But another villager said that neighbouring areas were in a far worse situation, as they had no protection at all.
Capt Fuller also said civilians could also be at risk from Congolese government forces, some of whom have not been paid for three months and are not getting adequate rations, according to reports.
"Some of them feel that they are going to pick up arms themselves against the civilian population that they are supposed to protect," he said.
The FDLR's presence in eastern DR Congo has been seen as a major factor in the region's instability since the Rwandan genocide 15 years ago.