ABUJA (AFP) - The Nigerian authorities have detained children in connection with the Islamist insurgency they are battling, but the government plans to release them under a peace gesture announced this week, a statement said on Thursday.

The statement from an adviser to President Goodluck Jonathan comes after widespread allegations of the Nigerian authorities carrying out indiscriminate arrests, unlawful detentions and extrajudicial executions in the fight against Islamist extremists.

It was issued because the government said it wanted to further clarify a Defence Ministry statement this week promising to release certain suspects held in connection with "terrorism," including all women. The military had made no mention of children.

"The order for the release of the detainees will be executed in phases," the statement from presidential adviser Doyin Okupe said.

"Concerning the first batch, the emphasis is on women and children who have been in detention on suspicion of involvement and/or connection with insurgency in some parts of the country." Mr Okupe did not respond to phone calls on Thursday for further details.

Islamist extremist group Boko Haram has previously demanded the release of female and child prisoners, though neither the ministry's nor government's statement made any mention of the demand.

Suspected Boko Haram leader Abubakar Shekau claimed in a recent video that the group was holding women and children hostage in retaliation for wives and children of its members being detained by the military.

The promise to release some of those detained comes as the military carries out an offensive in Nigeria's north-east, seeking to end a violent insurgency that has been ongoing since 2009.

Rights activists have long criticised Nigeria's military over its response to the insurgency, accusing it of major abuses.

The Boko Haram conflict is estimated to have cost 3,600 lives since 2009, including killings by the security services.