Footage of scores of girls kidnapped by Islamist militants in Nigeria demonstrates the "horror and barbarity" of the crime, Downing Street has said.
A video released by Boko Haram claims to show around 130 of the victims who were seized from a school last month.
The group's leader, Abubakar Shekau, is seen insisting that the girls will be held until all imprisoned militants have been freed.
There has been speculation that the comments could pave the way for a negotiated settlement - as Shekau has previously insisted the hostages would be sold.
Prime Minister David Cameron's spokesman said the Government had yet to examine the video in depth.
"It underlines the horror and the barbarity of the actions in the kidnapping of these girls," the spokesman said.
"It underlines the importance of working with and in support of the Nigerian authorities and Nigerian people in terms of trying to bring about the release of the kidnapped girls, which is so important."
Earlier, Mr Cameron refused to criticise the response of the Nigerian authorities, telling LBC radio that everyone was working "around the clock" to try to free the victims.
A British team including counter-terrorism and intelligence experts has arrived in Nigeria and held talks with counterparts and political leaders.
"It's a very difficult situation," the premier said. "It's an absolutely hideous, vicious, cruel, evil act, to take these more than 200 children into a forest.
"But it's a forest area three times the size of Wales... these things aren't easy to deal with.
"I don't think this is the moment to criticise, I think it's a moment to just get stuck in and see what more can we do to help and to make that offer."
The 17-minute Boko Haram video, obtained by French news agency AFP, showed three of the girls wearing the full-length hijab.
Two say they were Christian and have converted to Islam, while the other says she is Muslim.
"These girls, these girls you occupy yourselves with... we have indeed liberated them. These girls have become Muslims," Shekau says in the video.
There is no indication of where or when the footage was taken.
Former prime minister Gordon Brown, who has been visiting Nigeria in his role as United Nations special envoy for global education, said: "There will be worldwide condemnation of a new video showing Boko Haram cruelly and barbarically using 200 kidnapped girls to bargain for the release of prisoners and exploiting innocent young girls for political purposes.
''It is urgent that all religious leaders in every part of the world speak out against their perverted and twisted version of Islam, which involves forced conversions and the sale of girls as sex slaves.
"Meeting the Nigerian president in Abuja on Friday I was able to inform him of offers of military help now available from four countries.
"Countries around the world are also supporting the Safe Schools Initiative which has been launched to protect other schoolgirls from abduction and kidnap."
The Archbishop of Canterbury the Most Rev Justin Welby said : "This is a really complicated group to deal with and there are no simple solutions here. Obviously if the girls get freed, that is absolutely wonderful but that would only be one step in a long process that is going to be hugely difficult, of reconciliation."