Suspected Islamist extremists have abducted about 100 female pupils from a school in north-east Nigeria , but some later escaped from the back of an open truck, officials say.

The girls were abducted after midnight from a school in Chibok, on the edge of the Sambisa Forest, an insurgent hideout, said the Borno state police commissioner Tanko Lawan.

Gunmen killed a soldier and police officer guarding the school, then took off with at least 100 students, a state security service official said.

A local government official said he did not know how many of the girls had escaped but that many had walked through the bushes and back to Chibok. The girls were piled into the back of an open truck and, as it was travelling, some grabbed at low-hanging branches to swing off while others jumped off the slow-moving vehicle, he said. The two officials spoke on condition of anonymity because they were not authorised to give information to reporters.

All schools in Borno state were closed three weeks ago because of an increasing number of attacks by militants who have killed hundreds of students in the past year. But the young women ? aged between 16 and 18 ? were recalled to take their final exams, the local government official said.

Islamist extremists have been abducting girls to use as cooks and sex slaves.

Insurgents from the Boko Haram terrorist network are blamed for attacks that have killed more than 1,500 people this year alone. The group ? whose name means "western education is forbidden" ? has targeted schools, mosques, churches, villages and agricultural centres in increasingly indiscriminate attacks. They have also made daring raids on military barracks and bases.

The extremists are also accused of Monday morning's explosion at a busy bus station in Nigeria's capital that killed at least 75 people and wounded 141.