Nigeria's President Muhammadu Buhari has suggested that military operations against the Takfiri Boko Haram militants could be extended as the Abuja government is likely to miss a deadline to end the militancy.

In a message sent to an army top brass meeting in the northern state of Jigawa, President Buhari said on Monday that the self-imposed December 31 deadline was only "a guide."

"Let me emphasize that the time frame should serve as a guide and if exigency of multiple operations across the country advises a modification, the federal government will not hesitate to do so in order to address new flashpoints that are rearing their ugly heads in some parts of the country," Buhari said.

In August, the Nigerian president pledged to defeat the terrorists and gave the country’s military commanders until the end of December to meet that goal.

The Nigerian military has claimed a string of successes against the militants in recent months. But with just weeks left to the deadline there has been no let-up in the Boko Haram violence in Nigeria and neighboring countries.

Militant attacks and bomb explosions still remain widespread in Nigeria's northeast. The terror attacks are almost a daily occurrence in northern Cameroon, southeastern Niger and on the Chadian side of Lake Chad, where all four countries meet.

The latest presidential statement will likely be seized upon by Buhari's political opponents as backpedaling.

However, Buhari had earlier made it clear that he expected an end to "the main conventional attacks" by the deadline.

"What may not absolutely stop is the occasional bombings by the use of improvised explosive devices. We do not expect a 100 percent stoppage of the insurgency," the president said an interview with AFP news agency in September.

At least 20,000 people have been killed and more than 2.5 million made homeless since the beginning of the Boko Haram militancy in Nigeria in 2009. The militants have recently pledged allegiance to the Takfiri Daesh terrorist group, which is primarily operating inside Syria and Iraq.

Nigeria's neighbors, which have all suffered attacks by Boko Haram, set up a regional force to end the conflict.

The Nigerian president has made a multinational force of 8,700 central to his strategy in tackling the crisis.

Boko Haram's ringleader Abubakar Shekau has repeatedly threatened to attack countries that have joined forces against the militants.