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

In the 17-minute film, about 130 girls wearing full veils can be seen praying in an undisclosed location. Sitting on scrubland near trees, reciting the first chapter of the Qu'ran and holding their palms upwards in prayer, two girls say they were Christian but had converted.

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

In the video, the leader of the Islamist group, Abubakar Shekau, said he would release the girls in exchange for Boko Haram prisoners.

Speaking in Hausa and Arabic, Shekau restates his claim of responsibility made in a video released last Monday: "These girls, these girls you occupy yourselves with … we have indeed liberated them. These girls have become Muslims."

Speaking about his terms to end the kidnapping, he said: "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 added.

International efforts to trace the girls have widened in recent days. On Sunday, Israel joined the bid to find the hundreds of teenagers abducted from their dormitory in Nigeria's restive north-east.

Nigeria's president, Goodluck Jonathan, has accepted an offer of assistance from the Israeli prime minister, Benjamin Netanyahu.

Jonathan told Netanyahu: "Nigeria would be pleased to have Israel's globally acknowledged anti-terrorism expertise deployed to support its ongoing operations", according to the president's spokesman, Reuben Abati.

Britain, the US and France have already sent specialist teams and equipment to help Nigeria's military in the search concentrated in the remote north-east, which has been hit by five years of deadly violence.

Some of the kidnapped victims managed to flee their abductors. Science student Sarah Lawan, 19, told the Associated Press on Sunday that more young women who were seized could have escaped but they were frightened by their captors' threats to shoot them.

Speaking via telephone from Chibok, she said: "I am pained that others could not summon the courage to run away with me," she said. "Now I cry each time I come across their parents and see how they weep when they see me."

Boko Haram, whose name translates loosely from the Hausa language spoken widely in northern Nigeria as "western education is sin", has attacked schools, churches, government installations and, increasingly since 2009, civilians.

This year more than 1,500 people have been killed, despite a state of emergency imposed in three north-east states in May last year that was designed to put down the insurgency but has failed to stem the bloodshed.

The kidnapping of young girls and women has been used as a previous tactic, but the scale of these abductions – and threats from Shekau to sell the girls as slaves – has galvanised the international community into action.