The mother of one of the kidnapped Chibok schoolgirls being held by the Boko Haram terrorist group yesterday pleaded for Nigeria's president to free militants in exchange for the release of the 218 girls.

The appeal came after Boko Haram posted a video on Youtube showing dozens of the Christian schoolgirls, with a militant and one of the girls saying that "some" of them had been killed in military airstrikes while "about 40" had been married.

In the video, one of the kidnapped girls begged their parents to press Nigeria's government, led by President Muhammadu Buhari, to free detained Boko Haram members so the schoolgirls can be released in a prisoner swap.

Weeping, the girl's mother, Esther Yakubu, told the Associated Press: ''The government should just release the militants...All the girls that have been rescued have rescued themselves. Not any government has rescued them, no army rescued them."

Nigeria's defence ministry disputed the claim that some of the girls had died as a result of airstrikes. ''It is extremely difficult and rare to hit innocent people during airstrikes because the operation is done through precision attacks on identified and registered targets and locations,'' a spokesman, Brigadier General Rabe Abubakar, said in a statement.

Boko Haram seized more than 270 girls from their school in Chibok, north-east Nigeria, in April 2014, as part of a seven-year-old insurgency to set up an Islamic state in the north that has killed some 15,000 people and displaced more than two million.

Dozens of the girls managed to flee to safety in the initial melee, but more than 200 are still missing.

In the video published on social media, which was seen by Reuters on Sunday, a masked man stands behind dozens of girls.

"We want to send this message first to the parents of these girls for them to know that these girls are still with us, some of them, and secondly they should tell the Federal Government of Nigeria, to with immediate effect, release our imprisoned brothers," the man said.

"Some of the girls, about 40 of them with God's permission have been married, some of them have died as a result of bombing by the infidels," he said.

A veiled girl could be seen holding a baby. Parents have accused Boko Haram of having married off some of the girls against their will.

At the end of the video unidentified bodies could be seen on the ground.

"Military jets have killed some of the girls," said one of the girls, who was identified by her father, Yakubu Kabu, as his daughter called Dorcas.

"When I heard her voice, I realise she is my daughter," he told reporters in Abuja. "we are pleading with the government to help...The children are suffering. Some of them could be very sick‎."

Information Minister Lai Mohammed said in a statement the government "was on top of the situation" to free the girls.

"Since this is not the first time we have been contacted over the issue, we want to be doubly sure that those we are in touch with are who they claim to be," he said.

Army spokesman Rabe Abubakar was quoted as saying by PR Nigeria, an official government agency, that the military disputed the claims that the air force had hit the girls.

"We are nevertheless studying the video clips to examine [whether] the victims died from other causes rather [than] from the allegation of airstrike," he said.

Authorities said in May that one of the missing girls had been found and President Buhari vowed to rescue the others.

Boko Haram, which last year pledged loyalty to the militant group Islamic State, has kidnapped hundreds of men, women and children.

Under Buhari's command and aided by Nigeria's neighbours, the army has recaptured most territory once lost to Boko Haram, but the group still regularly stages suicide bombings.

Boko Haram has apparently split with Islamic State naming Abu Musab al-Barnawi two weeks ago as the group's leader for West Africa in a two-page interview in its weekly magazine.

But the previous figurehead Abubakar Shekau appears to have rejected the new role in another video published after Barnawi's appointment.

Additional reporting by Reuters.