More than 20 schoolgirls kidnapped by Islamist group Boko Haram in Nigeria in 2014 have been released, Nigeria's government confirmed on Thursday.

"The release of the girls...is the outcome of negotiations between the administration and the Boko Haram brokered by the International Red Cross and the Swiss government," a government statement said, confirming that 21 girls were freed. "The negotiations will continue."

Around 270 girls, most of them Christians, were taken from their school on April 14, 2014. Dozens have managed to escape, but more than 200 are still missing.

Their capture was part of Boko Haram's seven-year-old insurgency to set up an Islamic state in the north of Nigeria that has killed some 15,000 people.

More than 910 schools have been targeted by the Islamist group, whose name means "Western [or non-Islamic] education is a sin". At least 611 teachers have been deliberately killed and another 19,000 forced to flee. At least 1,500 schools have closed.

In a video released in May 2014, then-Boko Haram leader, Abubakar Shekau, said women and girls would continue to be abducted to "turn them to the path of true Islam" and ensure they did not attend school.

The fighting has sparked a largely unreported refugee crisis with an estimated 2.2 million people, including 1.4 million children, displaced. Only around 10 per cent are in government-recognised refugee camps where there is some schooling. The other 90 per cent are living with friends and family members with little or no access to education.

A video posted by Boko Haram in August this year showed dozens of the missing Chibok schoolgirls with a militant. One of the girls said that "some" of them had been killed in military airstrikes, while "about 40" had been married.

Additional reporting by Reuters.