MAIDUGURI, Nigeria — Twenty-one of Nigeria’s Chibok schoolgirls kidnapped by Boko Haram more than two years ago have been freed in a swap for detained leaders of the Islamic extremist group, the government and military said Thursday.

Some 197 girls remain captive, though it is not known how many of them may have died.

The freed girls, the first to be released as a result of government action, are in the custody of the Department of State Services, Nigeria’s secret intelligence agency, according to presidential spokesman Garba Shehu.

The government “wants the girls to have some rest, with all of them very tired coming out of the process,” Shehu said.

All but three of the schoolgirls were carrying babies, an aid worker who saw the girls in Maiduguri told The Associated Press. He spoke on condition of anonymity because he was not authorized to speak to the press. Many Boko Haram captives recently freed by military action have been shunned by their communities because they have come home pregnant or with babies from the fighters.

Their release was negotiated between the government and Boko Haram with the International Committee of the Red Cross and the Swiss government acted as intermediaries, said Shehu and a statement from the ICRC. Negotiations will continue for the release of the other students, said Shehu.

Four detained Boko Haram leaders were released Wednesday night in Banki, a town on the northeast border with Cameroon, said a military officer familiar with the talks who spoke on condition of anonymity because he is not authorized to speak to the press on the matter. The girls were flown by helicopter to Maiduguri, the northeastern capital of Borno state and birthplace of Boko Haram, he said.

“We are extremely delighted and grateful,” the Bring Back Our Girls movement said on Facebook. The group, which has campaigned within Nigeria and internationally for the release of the students, said it awaits the names of the released girls.