A long-awaited regional task force is set to begin raids on Boko Haram's last enclaves when the rainy season ends soon, the U.N.'s top official in West Africa said.

Nigerian and Chadian forces early this year forced the militant group, which has sworn allegiance to Islamic State, to cede large swathes of territory in northern Nigeria, undermining its six-year campaign to carve out a caliphate.

But some fighters have since regrouped and ramped up suicide attacks and guerrilla raids in the remote border areas around Lake Chad where Chad, Niger, Cameroon and Nigeria meet.

"They will take advantage of the end of the rainy season now to really go after them," said Mohamed Ibn Chambas, U.N. Special Representative for West Africa, in an interview on Wednesday. The rains in northeast Nigeria typically end in September but have lasted longer this year.

The 8,700-strong joint force, headquartered in Chad's capital N'Djamena with troops from Chad, Niger, Benin, Nigeria and Cameroon, was supposed to be fully functional in July.

But plans were not finalised until late August, and some observers have bemoaned a lack of progress since.

The African Union and the Lake Chad Basin Commission signed a memorandum of understanding in October giving final implementation guidelines and the United States has sent troops to provide intelligence and other assistance.

The expected joint raids will have to adapt to the changing nature of the enemy, which once attacked with hundreds of fighters aboard scores of vehicles but has been reduced to isolated bands, Chambas said.

"There are still remote areas where they are hiding and they need to be physically flushed out," he said.

Two such enclaves are Nigeria's Sambisa Forest, a vast former colonial hunting reserve, and the rugged mountains straddling the Nigeria-Cameroon border.

Individual national armies continue to battle Boko Haram but there has been little sign of joint operations for months. Chad's President Idriss Deby has indicated the force could begin operations this month.

Cooperation has sometimes been hampered by communication problems between English-speaking Nigeria and its francophone neighbors, but Chambas said the situation had improved.

He added that a purely military solution would not defeat Boko Haram, referring to the deeper causes of radicalism such as unemployment and climate change on the shrinking Lake Chad.

"You can't just physically eliminate Boko Haram and say the problem is gone," he said.

(Reporting by Emma Farge; Additional reporting by Aaron Maasho in Addis Ababa; Editing by Andrew Roche)