A file image taken from video by Nigeria's Islamic extremist network, shows the alleged missing girls abducted from the northeastern town of Chibok. An unknown number of girls kidnapped from their Nigerian boarding school by jihadists three years ago have been released, a government official said on Saturday. Picture: AP

Abuja,  Nigeria - Up to 50 Chibok schoolgirls were released by Islamist Boko Haram militants on Saturday after more than three years in captivity, a Nigerian government spokesman said, the largest group yet to be freed after years of tense negotiations.

The girls were among about 220 students abducted from a secondary school in the northeastern town of Chibok in 2014, sparking a global campaign #bringbackourgirls supported by then U.S. First Lady Michelle Obama and list of celebrities.

About 21 Chibok were released in October in a deal brokered by Switzerland and the International Red Cross, while a handful of others have escaped or been rescued. But about 195 were still missing.

"The government will soon release an official statement," a government spokesman told the Thomson Reuters Foundation with the exact number of girls freed remaining uncertain.

Last month President Muhammadu Buhari said in a statement that the government was "in constant touch through negotiations, through local intelligence to secure the release of the remaining girls and other abducted persons unharmed".

The girls were taken from a school in Chibok in the remote northeastern Borno state where Boko Haram has waged an insurgency aimed at creating an Islamic state that has killed thousands and displaced more than 2 million people.

For more than two years there was no sign of the Chibok schoolgirls but the discovery of one of the girls with a baby in May 2016 fuelled hopes for their safety, with a further two girls found in later months and the group released in October.

United Nations special rapporteurs have stressed that the Chibok girls are not the only ones who have suffered violence at the hands of Boko Haram.

At least 2,000 boys and girls have been kidnapped by Boko Haram since 2014, with many used as cooks, sex slaves, fighters and even suicide bombers, according to Amnesty International.

The use of children as suicide bombers by Boko Haram is also on the rise in the Lake Chad region with 27 such attacks recorded in the first three months of 2017 compared to nine for the same period in 2016, the U.N. children's agency UNICEF said.

Despite having lost most of the territory it held in 2015, Boko Haram continues to wage its insurgency, which is now in its eighth year. 

Thomson Reuters Foundation