Boko Haram has sent a "proof of life" video which shows 15 of the more than 200 girls abducted by Islamist fighters from the northeast Nigerian town of Chibok two years ago, CNN reported.

The video showed the girls wearing black hijabs in an unspecified location, stating their names, that they were taken from Chibok and the date of the recording -- December 25, 2015, the broadcaster said.

Two of the three mothers of the 219 schoolgirls still missing since the mass abduction on April 14, 2014 recognised their daughters on the video but another broke down as hers was not there. All three, however, identified all the girls, as did a classmate, who was at home on the day of the kidnapping.

The video is the first concrete indication that at least some of the girls are still alive since a previous video released publicly by Boko Haram in May 2014. Then, about 100 of the teenagers were shown in Islamic dress, reciting passages from the Koran after Boko Haram leader Abubakar Shekau said they had converted to Islam.

Nigeria's information minister Lai Mohammed told CNN the girls in the video appeared "under no stress whatsoever" and there had been "little transformation in their physical appearance".

The kidnapped girls of Chibok | Source: Reuters

But he declined to comment directly on the state of talks with Boko Haram, which has previously said it would release the girls only in exchange for captured fighters in Nigerian prisons.

"There are ongoing talks. We cannot ignore leads but of course many of these investigations cannot be disclosed openly because it could also endanger the negotiations," the minister added.

- Prisoner swap -

AFP understands from a go-between that in mid-January this year, members of Boko Haram made contact with the government, requesting talks about a possible prisoner swap.

The militants then sent five still photographs of some of the girls, also wearing black hijabs, who were identified by some parents as being among those kidnapped from Chibok. The government then requested more concrete proof in the form of a video, which was then sent.

Boko Haram head Abubakar Shekau | Source: Reuters

Parents of the 219 girls were on Thursday set to hold a prayer vigil at the Government Girls Secondary School in Chibok to mark the second anniversary of the kidnapping. Boko Haram seized 276 from their dormitories but 57 managed to escaped in the hours that followed.

Amnesty International's Nigeria director M.K. Ibrahim called for the release of all captives and said the Chibok girls symbolised "all the civilians whose lives have been devastated by Boko Haram". (President) Muhammadu Buhari's government should do all it lawfully can to bring an end to the agony of the parents of the Chibok girls and all those abducted," he added.

Malala Yousafzai | Source: Reuters

Malala Yousafzai, who was shot by Taliban extremists for going to school in Pakistan's north west frontier province, also wrote an open letter to parents of the girl on the Malala Fund Blog.

Here's the full text,

Dear mothers and fathers,

I write this letter with a heavy heart, knowing you have endured another year separated from your daughters. I think of you every day since we first met two years ago – and join millions of people around the world in praying for the safety and swift return of your girls.

As I did last year, I call on President Buhari of Nigeria – and everyone who can help rescue the Chibok girls – to act now. Would a president give up the fight for his own daughter? These girls are just as precious to their families.

Parents, thank you for having the courage to send your daughters to school. My dream is that one day they will come home, finish their education and choose their futures for themselves.

I pray for the day when you can embrace your girls again.

Your sister,

Malala.