Of the 219 girls still missing, the source said that only 104 were left in the captivity of the sect while about 114 had always been married off by top commanders and converted to Islam.
The Nigerian government on Thursday denied that the released Chibok girls were swapped with detained Boko Haram terrorists. The sources did not disclose the amount.
The country's Information Minister, Alhaji Lai Mohammed, said the girls were freed after "painstaking negotiations and trust on both sides".
While the return of the 21 girls is an encouraging sign, analysts point out that Boko Haram is still a potent force in the region with rival factions competing for control.
The NUJ is also asking the government not to relent in the fight against terror until those held captive by the sect regain their freedom.
Switzerland "facilitated contacts between Nigerian representatives and intermediaries of Boko Haram" after a request from Abuja, a Swiss government spokeswoman said.
He said in a statement released Thursday, "It is confirmed that 21 of the missing Chibok Girls have been released and are in the custody of the Department of State Services, DSS".
Over 200 shoolgirls were abducted by Boko Haram militants over years ago in Governmnet Secondary School, Chibok, Borno state.
The insurgency has claimed more than 20,000 lives and displaced 2.6 million people from their homes since Boko Haram took up arms against the Nigerian government in 2009.
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In the past days, the Nigerian military has been carrying out a large-scale offensive in the Sambisa forest, a stronghold of Boko Haram, which previous year pledged loyalty to the Islamic State militant group.
It sparked one of the biggest global social media campaigns, with tweeters like Michelle Obama using the hashtag #BringBackOurGirls.
After the 21 were released in northeastern Nigeria on Thursday, they were taken to the capital, Abuja.
Travelling from eastern Yola to Abuja, Goni Mutah, the father of Asabe Goni, who was 21 when she was kidnapped, was overjoyed to hear of his daughter's release, and said he had immediately phoned his relatives to share the news.
"When I saw them, I just hugged them", Hosea said. They were very happy to be free.
The spokesman said Mr. Buhari "welcomes the release of the girls" but cautioned Nigerians to remember that 30,000 Nigerians have been killed in the Boko Haram conflict. In April 2016, CNN obtained footage of the girls, allegedly recorded in December 2015 of 15 of the girls identifying themselves.
Some of the girls kidnapped from Chibok in the 2014 video.
In that recording, a militant says that 40 girls were married off. The Nigerian military has been conducting numerous raids on their territory, rescuing several other Chibok girls in the process.
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