Suspected Boko Haram gunmen on horseback killed at least 24 people in two separate attacks on villages in northeast Nigeria, military and vigilante sources said on Wednesday.

The gunmen opened fire and threw explosive devices in Kolori and Ba'ana Imam, in Damboa Local Government, Borno state, in the attacks on Monday evening.

Although nobody has claimed responsibility and the militants are rarely on horseback, the attacks bore the hallmarks of the Islamist group, which has killed thousands in its six-year-old bid to set up a state adhering to sharia law in the northeast.

More than 700 people have been killed in militant attacks since President Muhammadu Buhari took office on May 29, vowing to crush Boko Haram. Most of the recent bombings and shootings have been in Borno, the birthplace of the insurgency.

Telephone networks are poor in the area where the latest attacks occurred. A military source, who wished to remain unnamed, said details only emerged on Tuesday evening when people who escaped arrived in Biu, the nearest major town.

"The area is remote and about 24 people were killed," he said.

Usman Buka, part of a vigilante group formed to fight militants, said he went to the affected villages late on Tuesday and heard details of the attacks.

"I was told Boko Haram people came on horses, firing into houses and at fleeing villagers," Buka said by telephone from Biu, noting that the village of Kolori was targeted first.

"They killed 17 people, while seven people were injured. They went to Ba'ana Imam, a few kilometers (miles) away, and killed seven people," he said.

Boko Haram's insurgency in Africa's most populous country has displaced about 1.5 million people since 2009.

An army counter-offensive several months ago forced the militants to scatter but they have since returned with a strategy of bombings and shootings in public places such as markets and places of worship.

Buhari has been working with neighboring countries to set up an 8,700-strong regional military task force to fight the militants, who have carried out attacks in Chad,Cameroon and Niger in the last few months.