Nigeria's army said it had rescued at least a dozen kidnapped women and children held captive by the Islamic extremist group Boko Haram.

Military spokesman Colonel Sani Kukasheka Usman said the group was rescued as the army cleared Boko Haram camps on Monday in north-eastern Borno state.

The army did not say where the women and children had been kidnapped from or give their condition.

Hundreds of hostages have been freed from Boko Haram captivity this year but none of the 219 girls abducted in April 2014 from a school in Chibok was among those rescued.

Nigeria's homegrown Islamic extremist group has used dozens of girls and women in recent suicide bombings in Nigeria and neighbouring Chad, Cameroon and Niger, raising fears it is using kidnap victims.

More than 1,000 people have been killed since President Muhammadu Buhari was elected in March with a pledge to annihilate the militants, whose six-year-old uprising has killed a total of about 20,000 people.

At least 2.1 million people have been driven from their homes, some across borders.

Earlier this year, troops from Chad and Nigeria drove the extremists out of some 25 towns held for months in an area that Boko Haram had declared an Islamic caliphate aligned with the Islamic State group in the Middle East.

The insurgents have returned to hit-and-run attacks and suicide bombings.