At least 26 people were killed on Monday when two vehicles detonated an improvised explosive device in Nigeria's northeastern Borno state, the heartland of an Islamist insurgency, residents said.

The International Safety Organisation, which provides security to foreign non-governmental organisations in the northeast, said in an internal memo seen by Reuters that the vehicles moving between the towns of Rann and Gamboru Ngala hit an IED.

The blast led "to the death of 26 individuals and three injuries," it said.

Borno state police could not immediately comment.

Islamist insurgents Boko Haram and Islamist West Africa Province have been battling Nigerian security forces for over 15 years in the northeast and often use improvised explosive devices to target civilians and security forces.

Liman Tom, a traveller on the road, said the vehicles were badly damaged and survivors were rushed to hospital by soldiers and members of the civilian joint task force who arrived on the scene after the incident.

Abba Amma Muhammad, who saw the wreck of the car that was carrying his mother, blamed the incident on Boko Haram.

"I can not even recognize her remains," Muhammad told Reuters.