Twenty one of over 200 Nigerian schoolgirls kidnapped by Islamist militant group Boko Haram have been released after a prisoner was swapped for the girls by the Nigerian government, according to reports on Thursday. The girls were taken hostage from the northeast Nigerian town of Chibok in April 2014, triggering international outrage.

The swap deal was struck between Boko Haram, the International Committee of the Red Cross and the Nigerian and Swiss governments, presidential spokesman Garba Shehu said on Twitter. He added that negotiations were ongoing.

“It is confirmed that 21 of the missing Chibok girls have been released and are in the custody of the department of state services,” Shehu said in a statement. “The release of the girls... is an outcome of negotiations between the administration and the Boko Haram brokered by the International Red Cross and the Swiss government. The negotiations will continue.”

In May, one of the kidnapped Chibok schoolgirls was found by the Nigerian army. Sera Luka was reportedly from the northeastern town of Madagali, in the state of Adamawa.

Boko Haram had pledged allegiance to the Islamic State group, also known as ISIS, in 2015. It has been active in Nigeria since 2009. The group has killed over 30,000 people  and forced thousands to flee from their homes.