Nigeria has insisted it will not agree to a request to free imprisoned Islamic militants in return for the release of dozens of kidnapped schoolgirls.

Interior Minister Abba Moro said Boko Haram, the group holding the girls, was in no moral position to make the offer.

The group earlier released a video of the girls and suggested a swap, and a Nigerian official had said that all options were on the table.

Boko Haram snatched more than 200 girls from a school on 14 April.

About 50 children escaped, and it is not known how many are still being held.

The video released on Monday showed 136 girls, and was interspersed with militants explaining that they had "converted" to Islam.

Three of the girls - wearing full-length cloaks - are shown speaking. Two say they were Christian and have converted, while the other says she is Muslim.

The US state department said intelligence experts were closely examining the footage for clues to the girls' whereabouts.

Boko Haram leader Abubakar Shekau said on the video: "For those who have not accepted Islam, I swear to Allah we will never release them until after you release our brethren in your prison."

A man who is related to three of the girls said the video at first gave him hope, but then made him anxious and tearful.

"Maybe they are converted into another religion by force, so it truly is a kind of terrifying situation," said the man, who did not want to be named.

Earlier reports said some of the girls had been married off to their captors, and Abubakar Shekau had also threatened to sell some of them.

After the video was released, information ministry official Mike Omeri had suggested that "all options" were still being considered.

However, Mr Moro told the BBC that the government would not agree to any kind of swap deal.

"As far as this government is concerned, the option of [the] swap of innocent citizens with people who have taken arms against the country... is not on the table," he said.

Boko Haram, which means "Western education is forbidden", had previously said the girls should not have been at school and should get married instead.

The militants have been engaged in a violent campaign against the Nigerian government since 2009.

The government has faced heavy criticism of its response to the mass abduction.