Nigeria's President Muhammadu Buhari applauds as he welcomes a group of Chibok girls, who were held captive for three years by the millitant group Boko Haram, in Abuja. Picture: Bayo Omoboriowo/Presidential Office/Handout via Reuters

Abuja - Boko Haram militants freed 82 of more than 200 schoolgirls they kidnapped from the northeastern Nigerian town of Chibok in April 2014 in exchange for prisoners, the government said on Saturday.

Three years ago, the abduction of the girls from their secondary school by the jihadist group Boko Haram sparked global outrage and a celebrity-backed campaign #bringbackourgirls.

For more than two years there was no sign of the girls. But the discovery of one of them with a baby last May raised hopes for their safety, with a further two girls found in later months and a group of 21 released by the Islamist militants in October.

Nigeria thanked Switzerland and the International Committee of the Red Cross - who brokered the October release - for helping secure the freedom of the 82 girls after "lengthy negotiations", the presidency said in a statement.

Following this release, 113 of the Chibok girls are believed to be still in captivity.

Here are 10 key facts about the Chibok girls and Boko Haram:

* Since 2009, Boko Haram has waged an insurgency to carve out an Islamic state in northeast Nigeria that has killed at least 20,000 people and displaced more than two million.

* The most high-profile attack took place on April 14, 2014, when Boko Haram kidnapped 276 schoolgirls from a secondary school in Chibok in northeast Borno state. Around 50 of the girls escaped in the initial melee but 219 were captured.

* Nigeria's government and military, then under the command of former president Goodluck Jonathan, faced heavy criticism for their handling of the incident, with towns and cities across the nation witnessing protests.

* The kidnappings sparked a strong social media reaction, with the phrase #bringbackourgirls tweeted around 3.3 million times by mid-May 2014, and the global campaign which followed backed by then US First Lady Michelle Obama.

* Hope for the girls was briefly raised in April 2015 when the Nigerian military announced it had rescued 200 girls and 93 women from the Sambisa forest, northeast of Chibok. It was later revealed that the Chibok girls were not among them.

* One of the Chibok girls, Amina Ali, was rescued in May 2016. Held for months by the Nigerian government, she told her mother the girls were starved and resorted to eating raw maize, and that some had died in captivity, suffered broken legs or gone deaf after being too close to explosions.

* At least 2,000 girls and boys have been kidnapped by Boko Haram since the beginning of 2014, according to Amnesty International, which says they are used as cooks, sex slaves, fighters and even suicide bombers.

* Boko Haram used 27 children to carry out suicide attacks in West Africa in the first three months this year, almost surpassing the total of 30 child bombings during 2016, said the UN children's agency UNICEF.

* The militants split last year with one faction moving away from the group's established figurehead Abubakar Shekau over his failure to adhere to guidance from Islamic State to which Boko Haram pledged allegiance in 2015.

* The group of 21 girls freed in October have since been held in a secret location in the capital Abuja for assessment, support and debriefing by the Nigerian government. 

Thomson Reuters Foundation