The headmistress of a school in Nigeria has appealed to the government to do more to save teenage girls abducted by suspected Islamist militants on Monday.

Asabe Kwambula also called on the kidnappers - thought to be members of the Boko Haram group - to "have mercy on the students".

Officials said some 85 girls were still missing while 44 managed to escape.

Separately, Boko Haram has said it was behind Monday's bombing in the capital, Abuja, which killed at least 75 people.

The group's leader, Abubakar Shekau, made the statement in a video message sent to news agencies.

He also threatened further attacks, but made no mention of the schoolgirls' abductions, which took place just hours after the blast in Abuja.

'Save innocents'

Ms Kwambula said she had so far "registered" 32 students as having escaped from the kidnappers, adding they appeared to be unharmed.

"I am pleading with the government to secure the release of the children, to save the lives of these innocents," she told the BBC's Will Ross.

"I am with the parents, praying continuously for the teenagers' safe return."

Her plea came after the military admitted that most of the girls had not been freed as it earlier stated.

Correspondents say the raid on the boarding school in Borno state is a great source of embarrassment for the Nigerian authorities, who have been saying that their military campaign against the militants is succeeding.

Gunmen reportedly stormed the remote school in Chibok late on Monday, stealing food supplies and ordering the students on to lorries.

It is thought that the militants took the girls to forested areas near the Cameroonian border.

Some of the abducted later managed to jump off the trucks, while others ran from captivity during prayer time or while they were cooking.

The security forces have been working with vigilante groups and local hunters to find the schoolgirls.

The well-armed Boko Haram fighters have killed hundreds of civilians this year, slitting the throats of many of their victims, our reporter says.

Borno, Yobe and Adamawa states in north-east Nigeria have been under emergency rule since last May.

Militants from Boko Haram - which means "Western education is forbidden" in the local Hausa language - frequently target educational institutions.