Many of the FDLR rebels fled to Congo after the 1994 Rwandan genocide
Hutu rebels have retaken positions they lost in the eastern Democratic Republic of Congo, UN peacekeepers say.
The UN says it has reports that FDLR rebels captured several villages and a former military training school, days after Rwandan troops began to withdraw.
However, Congolese officials said the rebels made "hit-and-run" raids, denying it was a major regrouping.
Rwandan troops began withdrawing last Wednesday - five weeks after they crossed the border to attack the FDLR.
In January, the government in Kinshasa allowed thousands of Rwandan soldiers to enter eastern DR Congo to fight the remnants of the Rwandan Hutu militia.
Some of the FDLR rebels are accused of taking part in the 1994 Rwandan genocide, before fleeing across the border into DR Congo.
In a separate development, UN Secretary General Ban Ki-moon - on a visit to Rwanda - said achieving peace peace in the volatile region depended on co-operation between the governments in Kinshasa and Kigali.
He said he welcomed a plan by Rwanda's President Paul Kagame for the establishment of full diplomatic relations with DR Congo, speaking of his hope for a "new chapter" in relations between the two neighbours.
In eastern Dr Congo, however, a spokesman for the UN peacekeeping mission said on Sunday he had reports that the FDLR rebels had retaken several positions in the area.
But Congolese Information Minister Lambert Mende Omalanga said the rebels were carrying out raids rather than moving back.
"We didn't hear report that they are retaking the places. What our reports are saying is that they are conducting hit-and-run operations," the minister told the BBC's Focus on Africa programme.
"They [the rebels] don't remain in a place. When they come they loot, they frighten people and they take what they want to take and they go back because our boys are around."
The latest reports about the FDLR attacks will inevitably raise fears among Congolese civilians that their armed forces are failing to stand up to the rebels, the BBC's Mark Doyle reports from eastern DR Congo.
The FDLR's presence in eastern DR Congo has always been seen as a major factor in the region's instability.
The Congolese and Rwandans launched a joint offensive against the FDLR in January.
Thousands of Rwandan troops were deployed in North Kivu but they are deeply unpopular because, along with Uganda, Rwanda occupied eastern DR Congo between 1998 and 2002.
On-and-off fighting involving the FDLR, the army and other militias has forced more than one million people in North Kivu to flee their homes since late 2006.
There are also fears that exactly the same scenario could take place in the north-eastern of the country, our correspondent says.
Ugandan troops entered the region late last year, with Kinshasa's permission, to help tackle another rebel group - the Ugandan Lord's Resistance Army (LRA).
The rebels - who were chased out of Uganda - have been committing widespread massacres of Congolese civilians.
However, nationalist Congolese politicians in Kinshasa say the Ugandans should now leave because their job is done.
This is despite a widespread feeling among Congolese civilians in the region that the Ugandans should stay, our correspondent says.