The militant Islamic group Boko Haram has released a video purporting to show 15 of the schoolgirls it kidnapped two years ago in Chibok, northern Nigeria.

The footage, believed to have been filmed on Christmas Day last year, shows 15 girls dressed in black hijabs and talking to a cameraman off-screen. They identify themselves as among the 276 girls taken by Boko Haram militants in April 2014.

Jihadists overran the school in Chibok, Borno state, late in the evening on April 14. All schools in Borno had been forced to close due to increased fighting, but the girls had returned to complete their exams.

Armed men killed a policeman and soldier who were guarding the school and then forced entry into the dormitories, ordering the students to climb into open-backed trucks. Some managed to escape, but two years on 219 are believed to remain in captivity.

The video is the first possible sighting of the girls since Boko Haram released a video of them in May 2014.

According to Reuters, some of those filmed in the latest video have been identified by their parents.

Rifkatu Ayuba and Mary Ishaya said they recognised their daughters, Saratu and Hauwa. A third mother, Yana Galang, identified five of the missing girls, and told Reuters they "were looking very, very well".

"They were definitely our daughters... all we want is for the government to bring back our girls," she added.

In the clip, the girls said they were being treated well but wanted to return to their families. They pleaded with the Nigerian government to cooperate with their captors to secure their release.

Nigerian authorities have faced overwhelming criticism for their failure to find the girls over the past two years. President Muhammadu Buhari, elected over former President Goodluck Jonathan last May, has pledged to fight Boko Haram's brutal insurgency and made security a key pillar of his campaign. However, despite the Chibok case being brought to international attention through the #bringbackourgirls campaign and a number of false rumours of their release, the girls remain missing.

Boko Haram has since abducted around 2,000 children, including some 300 in a November 2014 attack in Damasak. A report released this week by UNICEF said incidents of Boko Haram child suicide bombings have increased rapidly over the last year. Children as young as 8, the majority of them girls, have been used to bomb schools and markets.

Some reports have suggested that at least some of the girls taken from Chibok have been brainwashed by their captors, and have carried out murders on behalf of the group. The May 2014 video claimed they had converted to Islam.

To mark the two year anniversary of the Chibok abduction, the UK government today reaffirmed its commitment to supporting Nigeria in its fight against Boko Haram and to helping find those kidnapped.

"Our thoughts are with the Chibok schoolgirls, their families and the thousands of other men, women and children who have been brutally abducted by Boko Haram. The abduction of the girls was a particularly horrific example of Boko Haram's barbaric crime," said Foreign Office Minister for Africa, James Duddridge, and International Development Minister, Nick Hurd, in a joint statement.

"We remain determined and steadfast in our support of Nigeria in its fight against Boko Haram... Tackling the root causes of global problems such as violent extremism, terrorism and poverty is not only the right thing to do, it is also firmly in the UK's own national interest," the statement added.

Former prime minister and UN Special Envoy for Global Education, Gordon Brown, said the missing girls are "now a symbol of our apparent weakness to protect young lives".

"Their parents still wake up each morning not knowing whether their daughters are alive or dead, married or single or violated as slaves. They deserve better."