At least 25 suspected Boko Haram insurgents were killed in clashes between soldiers and the Islamist militants in northeast Nigeria and five civilians were killed in fighting elsewhere in the region, a military source and residents said on Monday.

A ceasefire agreement between Boko Haram and the Nigerian government was expected to lead to the liberation of more than 200 school girls kidnapped by the militants six months ago, and talks were due to continue in neighboring Chad on Monday.

Boko Haram has not confirmed the truce and there have been at least six attacks over the weekend -- blamed by security sources on the insurgents -- that have killed several dozen people since the announcement of the ceasefire.

A government spokesman has said that the fighting on Sunday may be the work of criminal gangs in the lawless region.

An army officer, who requested anonymity, said the militants tried to enter the town of Damboa late on Sunday through Alagarno, a Boko Haram hideout, but soldiers fought them off.

"Our men gunned down 25 of the insurgents because they would have entered Damboa and unleashed more terror on the town that is just picking up from its ruins," the officer said.

He said an armored vehicle and some arms were recovered from the insurgents.

Damboa, a garrison town near the border with Cameroon, has been the site of fierce fighting between the militants and Nigerian forces for months. The insurgents sacked the town in July but were driven out by an army counter-offensive.

A member of pro-government Civilian Joint Task Force vigilantes, Mohammed Haruna, said of clashes on Sunday, "Two of our members came to (the town of) Biu this morning from Damboa and said the soldiers engaged Boko Haram yesterday and the battle lasted till about midnight."

Separately, Maiduguri resident Andrew Tada, said the insurgents killed five people in Gava, a hilly town in Gwoza Local Government Area not far from Damboa.

Tada said his brother in Gava was lucky to have escaped to the top of a mountain.

"My brother is still there now with other relatives, women and children," he told Reuters after speaking with his brother on the phone.

"They (the militants) came yesterday (Sunday) while people were scouting for food at the foot of the mountain. When the insurgents sighted our people, they pursued them and slaughtered five," Tada said.