The number of Nigerian schoolgirls still missing from recent kidnappings at a secondary school in Chibok has risen to 276 - 30 more than the previous estimate, police say.

Police Commissioner Tanko Lawan also said the number of girls who escaped from their captors had increased to 53.

Lawan said confusion over numbers occurred because students from other areas were brought into the Chibok Government Girls' Secondary School for final exams last month after other schools in the Borno state were shut because of attacks.

The schoolgirls were abducted by gunmen from the school in Nigeria's Borno state last Tuesday.

"The students were drawn from schools in Izge, Lassa, Ashigashiya and Warabe A. and that is why, after the unfortunate incident, there were various numbers flying around as to the actual number of girls that were taken away," Lawan said.

The army had estimated the number of missing girls at 129 but school officials said more than 200 students were taken during the attack.

Reports this week also indicated some had been forced into "marriage" with their abductors, who paid a nominal bride price equivalent to $12. Other reports that could not be verified said some had been taken across borders, to Chad, Cameroon and to an island in Lake Chad.

Girls who escaped say their captors identified themselves as fighters in the Boko Haram network, though the group has not claimed responsibility for the abductions.

Boko Haram, an armed group whose name means "Western education is sinful", is fighting what it calls Western influence and wants to form an Islamic state in Africa's largest oil producer country.

The kidnappings have fuelled protests in Nigeria's capital Abuja among parents and activists angry at government and security officials' failure to rescue the girls.

Abuja blast

Meanwhile, a blast in Abuja has killed at least a dozen people, witnesses and police sources say.

Thursday's car bombing hit the suburb of Nyanya, close to the site of a morning rush-hour bomb attack at a bus station on April 14 that killed at least 75 people.

Besides the dead, at least 19 people were wounded in the blast, the Associated Press news agency reported, quoting police sources.

A correspondent from the AFP news agency reported seeing nine bodies that had been brought from the scene of the attack, while a witness at the same hospital who requested anonymity reported seeing seven more bodies.

Boko Haram has claimed responsibility for the April 14 blast in Nyanya and threatened further attacks.

Thursday's blast came a week before Abuja was due to host the World Economic Forum on Africa, an annual gathering which brings together international leaders, policy makers and entrepreneurs.

Nigeria's government has announced a large security operation to protect the capital during the forum.