The Nigerian government has said it is in contact with Boko Haram after the armed group released a new video purporting to show the schoolgirls it kidnapped more than two years ago, and offering to trade the captives in return for the release of its jailed fighters.

Boko Haram released the video on Sunday showing dozens of the 276 pupils they seized from the Government Girls Secondary School in Chibok on the night of April 14, 2014. Fifty-seven managed to escape in the immediate aftermath of the mass abduction.

"They should know that their children are still in our hands," a Boko Haram fighter, holding an assault rifle and his head covered by a turban, says in the video, which was posted on YouTube.

The masked fighter delivers his demands standing in front of dozens of young women.

"The Nigerian government has issued a statement saying that they are on top of the situation. What they mean by that is they are in touch with people purported to be behind that video," said Al Jazeera's Ahmed Idris, reporting from Lagos.

"The government ... [is] being extra careful to be double sure that they are talking to the right people and they are also taking the time and care to ensure that these girls are not harmed," Idris said.

However, it is still "not clear who the Nigerian government is talking to", he added.

A senior Nigerian military official also released a statement on Sunday denying claims in the video that some of the Chibok schoolgirls had been killed in air raids.

'Killed by air strikes'

The masked fighter in the video claimed to be the successor to Abubakar Shekau, Idris said. During Shekau's seven-year leadership of Boko Haram, more than 20,000 people were killed in fighting and more than 2.2 million driven from their homes.

Earlier this month, the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) named Abu Musab al-Barnawi as Boko Haram's new leader, replacing Shekau.

Boko Haram has been waging a campaign against the Nigerian government for several years, battling what it calls Western influence.

"The speaker said military air strikes had killed many of the kidnapped Chibok schoolgirls, and he asked the parents of these girls to press the government to release the groups' fighters from prisons across Nigeria," Idris said.

"The speaker also said that 40 of the schoolgirls were married, some were killed by air strikes and there were several injuries. He also used the video opportunity to assure his comrades in prison and urged them to be resilient and assured them they would soon be freed," he said.

INFOGRAPHIC: Violence in Nigeria

The kidnapping has become a hot political issue in Nigeria, with the government and military criticised for their handling of the incident and their failure to rescue any of the girls.

About 2,000 girls and boys have been abducted by Boko Haram since 2014, with many used as sex slaves, fighters, and even suicide bombers, according to Amnesty International, the London-based human rights organisation.

In recent months, Boko Haram has increasingly used suicide and bomb attacks as the Nigerian military pushes the group out of territories they once controlled.

President Muhammadu Buhari has declared Boko Haram "technically" defeated, and said success in the campaign would be measured on the return of the Chibok girls and other abductees.

“There have been a lot of incursions from reports that we have seen, of our military making incursions successfully into areas hitherto held by the insurgents," Mike Omeri, former head of the government's Nigerian Information Centre, told Al Jazeera.

"In addition to that, a number of other strategies that may not necessarily be on the pages of newspapers, have been undertaken."