The more than 200 Nigerian schoolgirls being held captive have converted to Islam and will only be released in a prisoner exchange, Boko Haram said in a video seen by AFP on Monday that showed the girls wearing full veils and praying.

The 17-minute video featuring Islamist leader Abubakar Shekau was filmed in an undisclosed location.

The Nigerian government, however, rejected any demands for a prisoner swap, according to AFP.

The girls have been held hostage since Boko Haram besieged the Chibok Government Girls Secondary School on April 14, kidnapping 276 students. Police say 53 of the girls have escaped.

Shekau threatened in another video last week to sell the girls into slavery.

There also have been reports that some of the girls have been forced into marriage with their abductors, who paid a nominal bride price of $12, and that some have been taken across the borders into Cameroon, Chad or the Central African Republic.

The failure to rescue those who remain captive a month after being taken has attracted mounting national and international outrage. Last week, Nigeria appealed for international help in the search.

US hostage negotiators and other experts from Britain, France, China, Spain and Israel are expected to help in the search for the girls.

Fresh push for 'terrorist' designation

The United States designated Boko Haram a foreign terrorist organisation in November last year, making it illegal to contribute to the group. Washington also last year put a $7 million ransom on the head of Boko Haram leader Abubakar Shekau.

Britain, Nigeria’s former colonial ruler, and France have also branded the Islamists a "terrorist" group.

Nigeria and the United States are currently pushing for the UN Security Council to designate Boko Haram as a terrorist organisation across its 15-member ranks, which would mean that the group's assets and financing would be blocked globally. It would also impose worldwide travel ban and arms embargoes on individuals and entities associated with the group.

MEP Marietje Schaake has led recent calls for the EU to place the Nigerian extremists on its black list of terrorist organisations and has called for the development of a "clear strategy on how the EU can help stop the spiral of violence that has gripped Nigeria".

US Defense Secretary Chuck Hagel cautioned on Sunday that it is “going to be very difficult” to find the missing girls. In an interview with the ABC news show “This Week” he said: “It’s a vast country ... But we’re going to bring to bear every asset we can possibly use to help the Nigerian government.”

Democratic US Senator Charles Schumer of New York has called for the offer of a reward – including refugee status – for anyone who is not a terrorist who provides help leading to the girls’ rescue.

Africa’s most populous nation of 170 million has almost equal numbers of Christians and Muslims, but dissent between the two communities has been on the rise – exacerbated by Boko Haram's deadly attacks and its five-year-old Islamist insurgency that has killed thousands of both Muslims and Christians and has driven some 750,000 people from their homes.

Boko Haram claimed two rush-hour bomb blasts that killed about 100 people and wounded more than 200 in the capital Abuja last month.

(FRANCE 24 with AFP and AP)