The FDLR has been in DR Congo for almost 15 years
Rwandan troops have entered eastern Democratic Republic of Congo for a joint operation with the Congolese against a Rwandan Hutu militia.
A UN spokesman told the BBC about 2,000 Rwandan troops had crossed the border.
DR Congo and Rwanda agreed last month to take joint action against the FDLR militia, whose leaders have been linked to the 1994 Rwandan genocide.
Correspondents say the FDLR's presence in eastern DR Congo lies at the heart of the region's instability.
Rwanda twice invaded its much larger neighbour during the 1990s, saying it was pursuing the Democratic Forces for the Liberation of Rwanda (FDLR).
But analysts say much of the fighting is also motivated by eastern DR Congo's rich mineral resources, which all sides have been accused of plundering.
The BBC's Thomas Fessy in the Congolese capital, Kinshasa, says diplomatic and UN sources fear a humanitarian disaster because of a possible lack of military planning and consultation with the international community.
Tanks and trucks
Jean-Paul Dietrich, a spokesman for the UN peacekeeping mission in DR Congo, told our correspondent that Rwandan troops had entered North Kivu province near the city of Goma and were heading for the Tutsi-rebel town of Rutshuru.
RWANDA IN DR CONGO
1994: After Rwanda genocide, Hutu rebels move to DR Congo (then Zaire)
1997: Rwanda invades DR Congo to deter cross-border rebel raids
1998: Rwanda invades again. At least four other nations intervene
2002: Rwandan military leaves DR Congo
2003: "Africa's world war" declared over
2008: Congolese Tutsi rebel offensive on Hutus
2009: Rwandan military enters DR Congo
Congolese troops have reportedly also been moving tanks and trucks full of ammunition early on Tuesday from Goma north towards Ruthsuru.
Our correspondent says it is not yet clear why the twin force is moving to the town, which is held by forces of the Congolese Tutsi rebel CNDP (National Congress for the Defence of the People).
But he points out the CNDP declared a ceasefire last week and offered to join with Congolese government troops against the FDLR, which is estimated to be more than 6,000-strong.
Action against the Hutu group has been a key demand of the CNDP, which seized swathes of territory last year.
Congolese Information Minister Lambert Mende Omalanga told the BBC's Focus on Africa programme that the operation would last between 10 and 15 days.
He said the Rwandan troops would only observe the operation.
"We need their full co-operation, to have these people [FDLR], when disarmed, back home."
He also said that a disarmament operation for fighters from both the CNDP and pro-government militias had been launched in Goma on Tuesday.
Our correspondent says more Rwandan troops are gathering in Gisenyi on the Rwandan side of the border but it is not clear when or if they will cross over.
Rwandan Information Minister Louise Mushikiwabo told Reuters news agency the Rwandan forces would operate under Congolese command.
FDLR spokesman Calixte Mbarushimana told the BBC that the Rwandan government should talk to them, rather than use force.
"It's a real pity that Rwanda wants to solve political problems with weapons," he said from his base in France.
On-and-off fighting involving the CNDP, FDLR, the army and pro-government militias has forced more than one million people in North Kivu to flee their homes since late 2006.
Some 250,000 people have been displaced since August 2008, when the CNDP rebels led by Gen Laurent Nkunda resumed fighting with the Congolese army.
Before last month's deal was signed, the UN accused Rwanda and DR Congo of fighting a proxy war in the region - with Rwanda backing Gen Nkunda and DR Congo of working with the FDLR.
All sides have also been accused of plundering eastern DR Congo's rich mineral resources - gold, diamonds, tin and coltan, used in mobile phones.
Some 800,000 Tutsis and moderate Hutus were killed during the 1994 Rwandan genocide, before Tutsi rebels led by current President Paul Kagame took control of the country.