Congo's president assured Secretary-General Ban Ki-moon on Wednesday that his army is ready to take military action with U.N. troops against a rebel group in that failed to surrender by a Jan. 2 deadline.

U.N. spokesman Stephane Dujarric said Ban called for "decisive action" against the Democratic Forces for the Liberation of Rwanda in a phone conversation Wednesday with Congo's President Joseph Kabila.

The rebel group, known as the FDLR, was formed by extremist Rwandan Hutus who took part in the 1994 genocide and then fled across the border into Congo.

A senior U.N. official said the U.N. has prepared "a joint directive" authorizing the operation which it expects Kabila's government to sign "very soon."

The official, who spoke on condition of anonymity because talks were private, would not say when military action will start, but it is likely to begin soon after the document is signed.

Congolese support is critical to the success of the military operation because once the rebels are defeated Congolese forces need to move in quickly to restore security and the government must start providing services, the official said.

Dujarric said Monday that the Congolese army and the U.N. peacekeeping mission have already prepositioned their troops and were ready to go.

The operation could take months because the estimated 1,500 FDLR combatants don't wear uniforms and live with their families in large areas of North Kivu, South Kivu and northern Katanga provinces, mingling with the civilian population, the official said.

Eastern Congo is home to a myriad of armed groups and militias, many vying for control of the region's vast mineral resources. Like the FDLR, they earn money from illegal sales of diamonds, gold and other precious metals.

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 the Intervention Brigade and authorizing the use of unarmed drones on a trial basis for intelligence gathering in the east.

Since then, government forces and U.N. peacekeepers have defeated one of Congo's main rebel groups, the M23.

The U.N. official said peacekeepers have been reaching out to the FDLR warning of an offensive and urging the fighters to surrender and leave with their families, but the rebels appear to be preparing for combat.

Congolese support is critical to the success of the military operation because once the rebels are defeated Congolese forces need to move in quickly to restore security and the government must start providing services, the official said.

The official said U.N. and Congolese troops on Monday attacked rebels from the National Liberation Forces, an ethnic Hutu rebel group that holds territory in eastern Congo, to clear the way for the offensive against the FDLR.