LAGOS, Nigeria (AP) — Nigeria’s military cannot meet the president’s December deadline to crush Boko Haram’s Islamic uprising, and Nigerians must expect suicide bombings to continue, a government spokesman said Thursday.

Air Commodore Yusuf Anas of the Center for Crisis Communication said the deadline “may be unrealistic” and warned Nigerians not to view December as a “sacrosanct date when all suicide bombings will end.”

The 6-year-old uprising already has killed 20,000 people and driven 2.3 million from the homes.

“The timeline on when to stop the insurgents from activating sleeper cells and detonating bombs into soft targets in any part of the country, especially in the frontline states, is therefore not tenable,” Anas said.

Forces from Nigeria and neighboring Chad earlier this year drove the extremists out of areas in which they had proclaimed an Islamic caliphate. Recently, the Nigerian Air Force and ground troops have reported destroying numerous Boko Haram camps and freeing more than 1,000 kidnap victims.

In June, President Muhammadu Buhari ordered the military to crush the insurgency by December, but the extremists have pushed back with village raids and urban suicide bombings that have killed more than 1,500 people.

They have also continued to strike in neighboring countries. On Wednesday night, suspected Boko Haram militants raided a town in southeast Niger, killing 18 people and kidnapping a 3-year-old girl, according to a statement read out on state TV Thursday night.

Last month, Buhari told the commander of the U.S. Africa Command, Gen. David Rodriguez, that improved training, weapons, logistics and welfare had put Nigerian forces in a stronger position.

Boko Haram was named the world’s most deadly extremist group in the Global Terrorism Index last week, with 6,644 deaths attributed to it in 2014 — more than any other extremist group.

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Associated Press writer Dalatou Mamane contributed to this report from Niamey, Niger.