The Nigerian military is joining the search for at least 100 teenage girls abducted from a school in Chibok, Borno State.
It is thought that Islamist militant group Boko Haram took them to a forest near the Cameroonian border.
The air force, army, police and local volunteers are all involved in the search, officials say.
The BBC’s Hausa Service says the group has kidnapped civilians in the past – usually women to work as sex slaves.
Gunmen reportedly arrived at the school in Chibok, a remote area of Borno State, late on Tuesday, and ordered its teenage residents on to lorries.
A local politician said about 50 army soldiers had been stationed near the school ahead of annual exams, but were apparently overpowered.
Local residents reported hearing explosions followed by gunfire.
“Many girls were abducted by the rampaging gunmen who stormed the school in a convoy of vehicles,” local education official Emmanuel Sam told the AFP news agency.
A girl, who managed to escape and did not want to be named, told the BBC that she and fellow students were sleeping when armed men burst into their hostel.
The girl said she and her schoolmates were taken away in a convoy, which had to slow down after some of the vehicles developed a fault, at which point 10 to 15 girls escaped.
“We ran into the bush and waited until daybreak before we went back home,” she said.
Nigerian media reported that two members of the security forces had been killed, and residents said 170 houses were burnt down during the attack.
The militants know the terrain well and the military has had only limited success in previous efforts to dislodge them from their forest hide-outs in the past.
It will be hard for any rescue effort to succeed without further endangering the girls’ lives, the BBC’s Nigeria correspondent Will Ross says.
Boko Haram is a fierce critic of Western-style education, and its militants frequently target educational institutions.
This year, the group’s fighters have killed more than 1,500 civilians in three states in north-east Nigeria, which are currently under emergency rule.
The government recently said that Boko Haram’s activities were confined to that part of the country. However, bombings blamed on the group killed more than 70 people in the capital city of Abuja on Monday.