Gunmen burst into a school in northeastern Nigeria and abducted students as they were sitting exams on Monday morning, police said.

The attackers fired sporadically as they charged into Government Day Secondary School in the town of Lassa, Borno State police spokesperson ASP Nahum Kenneth Daso said.

The military, police and other agencies were searching nearby forests to try and rescue the students who had been sitting their National Examinations Council exams, Daso said. The test is typically taken by 16-and 17-year-olds.

Officials were still trying to work out how many students were missing, as some remained unaccounted for following the attack that started around 9 am (0800 GMT), Daso said.