Many of the FDLR fled to DR Congo after the 1994 Rwandan genocide
Some 250,000 people in the Democratic Republic of Congo have been displaced following an operation to flush out Hutu rebels, aid agency Oxfam has said.
The joint operation against the rebels earlier this year was hailed as a great success by both Rwanda and DR Congo.
But now that the better-trained and equipped Rwandan army has left DR Congo, the Hutu militia is reportedly re-emerging from the forests.
Oxfam says various armed groups are now attacking civilians in the east.
Marcel Stoessel, Oxfam's country director in DR Congo, said the continued insecurity in North Kivu Province was making it difficult to deliver aid to those displaced.
"There is widespread looting, burning of villages and an unacceptable peak of sexual violence," he told the BBC.
Tens of thousands of people are fleeing from around the town of Kanyabayonga towards Lubero, a more populated area where they felt safer, he said.
"Oxfam is very worried that continued military operations are having a serious effect on the people who've had to flee their homes," he said.
On-and-off fighting involving the Hutu FDLR militia, the army and other militias has already displaced more than one million people in North Kivu since late 2006.
The BBC's Africa analyst Mary Harper says reports from the area indicate that it is members of the Congolese army and the FDLR militia that are attacking civilians, each accusing them of supporting the other side.
The fact that Congolese soldiers have not been paid for the past three months adds to the problem.
They are hungry, frustrated and probably terrified of attack from the many armed groups in the region, she says.
It seems the FDLR has not been crushed at all, she adds, rather that it engaged in a tactical retreat, vanishing for a few weeks only to reappear as a determined fighting force.
The FDLR's presence in DR Congo lies behind years of unrest in the region.
Some of the group's leaders are accused of taking part in the 1994 Rwandan genocide, in which some 800,000 ethnic Tutsis and moderate Hutus were slaughtered.
After the 1994 genocide, many of those responsible crossed into DR Congo as Tutsi rebels took power in Rwanda.
Rwanda has twice invaded DR Congo, saying it wants to stop the FDLR from staging attacks.