Up to 2,000 Rwandan government troops have entered the Democratic Republic of Congo, UN peacekeepers have said.
The joint Rwandan-Congolese operation, believed to be aimed at disarming the Democratic Forces for the Liberation of Rwanda (FDLR), began on Tuesday.
The Rwandan troops,are believed to be close to Rutshuru, a town which lies 80km northeast of Goma, the capital of the eastern province of North Kivu.
Lambert Mende, a government spokesman, said: "They are observers ... The operations to disarm the FDLR are planned to last 10-to-15 days."
Monuc, DR Congo's UN peacekeeping force, is not taking part in the security operation, which was agreed by the Congolese and Rwandan governments on December 5.
Ignace Murwanashyaka, the chairman of the FDLR, said on Tuesday that his forces are aware of the move.
"It's confirmed but we are not afraid ... So far there have been no clashes with our men," he said.
The move comes after a dozen dissident commanders of the National Congress for the Defence of the People (CNDP), a Tutsi rebel force allegedly backed by Rwanda, declared the end of their own war against Congolese government troops.
The CNDP's offensive in North Kivu against Congolese government forces, which began in October, led to more than 250,000 people fleeing their homes.
The commanders said they are willing to join Congolese and Rwandan government forces fighting the FDLR, in order to protect the Tutsi population from Hutu fighters.
Laurent Nkunda, the leader of the CNDP, has not made any comment on the dissidents' announcement that his group's war against Congolese forces has ended.
Al Jazeera’s Yvonne Ndege in the eastern Congo said: "If Rwanda is successful, it would be of great help to the Congo because it will also assist in pacifying General Nkunda who has been leading an 800-rebel strong insurgency in the eastern Congo for the last two years.
"Nkunda has always argued that he has been trying to get rid of the FDLR for the Congolese government has not done enough with the group and he has wanted and has insisted that the government get rid of the FDLR. Now, if Rwanda is successful against the FDLR, that effectively puts Nkunda out of business."
The Congolese government has promised to tackle Hutu fighters on several occasions, but has so far failed to dislodge them.
The CNDP says that this is one of the main reasons it was forced to take up arms again.
Several Congolese opposition leaders in the Mai-Mai, a Congolese Resistance Patriots (Pareco) movement, have also joined the fight against the FDLR.
Many of the Rwandan Hutu fighters fled into DR Congo in 1994 after taking part in the slaughter of an estimated 800,000 Tutsis and moderate Hutus.
The Hutu groups were also involved in the 1998-2003 war in DR Congo, which has led to the deaths of over five million people and displaced over one million.