More than 1,500 Rwandan troops crossed the border into eastern Congo on Tuesday to join Congolese forces in an effort to oust Hutu rebels who participated in Rwanda's genocide and have long been at the heart of the region's conflict, officials said.

Congolese government spokesman Lambert Mende said the Rwandan forces arrived Tuesday morning and that the joint military operations would last 10 to 15 days.

"We have officially asked the Rwandan army to participate in the disarmament operations of the Interahamwe (Hutu militia) which have begun," Mende said.

A Western diplomat, who spoke on condition of anonymity because of the sensitivity of the matter, said there was concern that the Rwandan Hutu FDLR militia might retaliate against civilians.

In a rare move, Congo and Rwanda have agreed to step up efforts against the Rwandan Hutu FDLR militia that has long destabilized the region. Still, neither country has been able to eradicate the Hutu rebels since they fled to Congo in 1994.

The Hutu fighters, who helped carry out the genocide in which more than 500,000 ethnic Tutsis and moderate Hutus were killed, have remained in Congo untouched, heavily armed, and in control of lucrative mines in remote hills and forests.

The militia has terrorized civilians, given Tutsi rebels a reason to fight and also are the reason why Rwanda invaded Congo previously in 1996 and 1998.

The U.N. mission in Congo said it was not associated with the operations but confirmed that the Rwandan forces had entered Congolese territory. U.N. peacekeeping spokesman Lt. Col. Jean-Paul Dietrich said between 1,500 and 2,000 Rwandan soldiers had crossed the border.

Louise Mushikiwabo, the Rwandan minister of information, said all forces were under the command of the Congolese national army, known by its abbreviation FARDC.

"This is the result of recent intense and sincere efforts — diplomatic, military and other — by various stakeholders to bring peace and stability to the region," Mushikiwabo said. "The government of Rwanda has played its part in a consistent and sincere manner. There is new momentum and the government of Rwanda is pleased the fundamental obstacle to stability for the last 15 years, i.e. the FDLR, is finally being tackled."

U.N. envoy Olusegun Obasanjo met in Kinshasa with Congo's President Joseph Kabila who briefed him on the ongoing joint Congolese-Rwandan operations against the FDLR. U.N. spokeswoman Michele Montas said in New York that the operations were expected to continue for "some weeks."

The Rwandan Hutus fled to Congo in 1994 and some lived in overflowing refugee camps around Goma. By 1996 their leaders launched an insurgency and began carrying out cross-border attacks into Rwanda, killing more Tutsis.

Fed up, Rwanda attacked the camps and drove on to Congo's capital, Kinshasa, installing late Congolese rebel leader Laurent Kabila as president in 1997.

Eager to prove his independence, Kabila in 1998 expelled the Rwandan Tutsis who brought him to power. Three days later, Rwanda organized another Congolese rebellion, and along with Uganda, seized eastern Congo in a war that drew in half a dozen African nations and lasted until 2002.

Since then, Congo has formed a unity government that gave top posts to rebels. Kabila's son Joseph won historic elections in 2006.

Former Rwandan-allied Tutsi rebels such as Laurent Nkunda were integrated into the army, but expressed frustration over the government's hesitancy to go after the Hutu militias who had served as their de facto allies during the war. The former general quit the army in 2004 and launched a rebellion.

Nkunda has used the threat posed by the Hutu rebels to justify carving out his own fiefdom in the mineral-rich east. But his critics contend he is more interested in power and Congo's mineral wealth.

Fighting between Nkunda's rebels and the government has left about 250,000 people displaced since August, according to the U.N. A decade of insecurity had already left nearly 800,000 others homeless in North Kivu — a startlingly high number for a provincial population of 6 million.