The Nigerian army says it has retaken the north-eastern town of Gwoza, believed to be the headquarters of militant Islamist group Boko Haram.

"Several terrorists died while many are captured," a tweet from the defences forces said.

Mopping-up operations are ongoing in and around the town, the military said.

News of the recapture comes a day before presidential elections, which were postponed by six weeks because of the offensive against Boko Haram.

Thousands of people have been killed since 2009, when the group began its insurgency to create an Islamic state.

'Major milestone'

Since a regional force began helping Nigeria take on the insurgents this year, many towns have been retaken.

The most recent military briefing said that only three districts, including Gwoza, remained under Boko Haram's control, down from 14.

After his fighters captured Gwoza in August 2014, the Boko Haram leader declared a caliphate in areas under his control.

"Troops this morning captured Gwoza destroying the Headquarters of the Terrorists self-styled Caliphate," a tweet from Nigerian armed forces said.

BBC Africa security correspondent Tomi Oladipo says capturing what Boko Haram called its headquarters is a major milestone for the Nigerian army.

Gwoza is not far from Chibok, where Boko Haram kidnapped more than 200 schoolgirls from a boarding school last April.

Our reporter says Gwoza was one of the places where the militants were rumoured to be hiding the girls, who are yet to be found.

Boko Haram promotes a version of Islam which makes it "haram", or forbidden, for Muslims to take part in any political or social activity associated with Western society.

Earlier this month, the group pledged allegiance to Islamic State militants, who control large parts of Syria and Iraq and are also active in Libya.