Army spokesman Onyema Nwachukwu said there was "credible information" the jihadists had "infiltrated some villages" near the Borno state capital Maiduguri.

"They have clandestinely been going about threatening innocent residents, warning them to vacate their homes," he said in a statement.

A member of the civilian militia told AFP earlier this week that Boko Haram had warned people in Jakana village to leave by Wednesday as they were going to launch a major attack.

On Wednesday, the army stepped up stop and search operations on the main road heading west out of Maiduguri, on which Jakana lies.

Warnings are also reported to have been given to locals in Mainok, which is on the same route leading to Damaturu, the capital of neighbouring Yobe state.

Nwachukwu described the threats as "psychological war" and urged residents to report suspicious activity.

Nigeria's government and military has repeatedly claimed the Islamist militants have been weakened to the point of defeat but attacks have persisted.

On Wednesday, the United Nations said more than 30,000 people had fled their homes in the Baga area of northern Borno after attacks from an Islamic State group-backed Boko Haram faction.

The attacks were the latest against military installations and soldiers that have increased since July last year. The military has rejected suggestions the jihadists are resurgent.