The Nigerian Islamist militant group Boko Haram has offered to trade more than 200 kidnapped schoolgirls for prisoners.

Boko Haram has released a new video claiming to show the missing Nigerian schoolgirls, alleging they have been converted to Islam and would not be released until all militant prisoners were freed.

The terrorist group's leader initially said he would sell the girls at the market but in the latest video says he will exchange the girls for every militant prisoner in Nigeria.

A Nigerian government official has said "all options" were being considered to secure the girls' release.

In the video the leader of Boko Haram, Abubakar Shekau, speaks for 17 minutes before showing what he says are about 130 of the girls, praying in an undisclosed rural location.

A total of 276 girls were abducted on April 14 from the north-eastern town of Chibok, in Borno state, which has a sizeable Christian community. Some 223 are still missing.

What is Boko Haram? Boko Haram, which means "Western education is sinful", is loosely modelled on the Taliban movement in Afghanistan.

Boko Haram, which means "Western education is sinful", is loosely modelled on the Taliban movement in Afghanistan. The group considers all who do not follow its strict ideology as infidels, whether they are Christian or Muslim.

The group considers all who do not follow its strict ideology as infidels, whether they are Christian or Muslim. It demands the adoption of Sharia law in all of Nigeria. Read more

Speaking in Arabic and the local language Hausa, Shekau restates his claim of responsibility made in a video released last Monday and said the girls had converted to Islam.

"These girls, these girls you occupy yourselves with ... we have indeed liberated them. We have indeed liberated them. Do you know we have liberated them? These girls have become Muslims," he said.

The militant leader said that Boko Haram's brothers in arms had been held in prison for up to five years and suggested that the girls would be released if the fighters were freed.

"We will never release them (the girls) until after you release our brethren. Here I mean those girls who have not submitted (converted to Islam)," he said.

Nigeria's interior minister Abba Moro rejected freeing any prisoners to secure the girls' release.

"The issue in question is not about Boko Haram ... giving conditions," he said.

West offers surveillence aircraft to assist search

The abduction has sparked worldwide outrage, with several nations offering military and intelligence assistance to locate the girls.

Diplomats, aid workers, defence officials and other experts from Britain and the United States have been working with the Nigerian government since Saturday.

US experts assisting the search for the kidnapped Nigerian schoolgirls are closely examining a new video for clues to their whereabouts.

Sorry, this video has expired Boko Haram release video of kidnapped girls

"Our intelligence experts are combing through every detail of the video for clues that might help ongoing efforts to secure the release of the girls," state department spokeswoman Jen Psaki said.

"We have no reason to question its authenticity."

Nigerian president Goodluck Jonathan says he is optimistic about finding those still held hostage, citing international support, with Israel the latest country to offer help.

Israeli prime minister Benjamin Netanyahu did not give details of the proposed assistance but Mr Jonathan confirmed the offer to send a team of counter-terrorism experts was made during a phone conversation.

A viral social media campaign calling for the schoolgirls' return has seen public figures such as United States first lady Michelle Obama tweeting with the hashtag #BringBackOurGirls.

On Sunday, French president Francois Hollande called on African leaders to hold a summit in Paris on security in west Africa.

The leaders of at least five African countries - Nigeria, Chad, Cameroon, Niger and Benin - may take part, a source close to the president said.

AFP/ABC