The U.N. peacekeeping force in Congo and the Congolese army will launch a military offensive against a rebel group that failed to surrender by a Jan. 2 deadline set by two regional groups, the United Nations said Monday.

U.N. spokesman Stephane Dujarric said both forces are preparing for the operation against the Democratic Forces for the Liberation of Rwanda, known as the FDLR, which was formed by extremist Rwandan Hutus who took part in the 1994 genocide and then fled across the border into Congo.

He said the Congolese army and the U.N. mission, known as MONUSCO, have developed a joint military plan for the operation and MONUSCO has already prepositioned its troops and equipment.

Dujarric said the top U.N. envoy in Congo, Martin Kobler, updated the Security Council by videoconference Monday morning on the FDLR and the security situation in the country. He did not say when the military offensive will begin.

Kobler has said the key to peace in eastern Congo is to disarm the estimated 1,500 combatants still fighting for the FDLR.

Eastern Congo is home to a myriad of armed groups and militias, many vying for control of the region's vast mineral resources. While the conflict is mainly a spillover from Rwanda, it also includes rebel groups from Burundi and Uganda.

In February 2013, the Congolese government and 10 other African nations, including Rwanda and Uganda, took the most concerted action to bring peace to Congo by signing an agreement not to interfere in each other's internal affairs or host armed groups.

The Security Council followed up by beefing up the U.N. force in Congo with an Intervention Brigade to undertake offensive operations against armed groups and authorized the use of unarmed drones on a trial basis for intelligence gathering in the east. Since then, government forces and MONUSCO troops including the Intervention Brigade have defeated one of Congo's main rebel groups, the M23.

In early July, the Southern African Development Community, the region's 15-nation organization, and the International Conference on the Great Lakes Region set a deadline for the FDLR to fully and unconditionally surrender by Jan. 2.

Dujarric said there had been no significant surrenders of FDLR combatants since the initial surrenders in June.

Since FDLR fighters are dispersed and immersed in local areas, he said, "it will take time to achieve the results we want, and various tactics will be used which may not necessarily lead to the immediate or large-scale blows to the armed groups."

Dujarric stressed that the FDLR's defeat requires more than military action.

"Military operations have a potential to weaken ... but not to eliminate the FDLR," he said. "The military solution needs to be part of a larger comprehensive strategy that includes a number of non-military steps," including dialogue.