Kano - Nigeria on Friday said three servicemen were killed in a military helicopter crash in the restive northeast, while Boko Haram rebels raided two more towns and vigilantes and hunters clawed back a key militant stronghold.
The second crash in a week happened late on Thursday in Yola, the capital of Adamawa, which is one of three states that has been under emergency rule since May last year.
The military said the aircraft involved was a ground attack helicopter on an armed patrol.
"The crew of three was lost in the ill-fated accident," a statement said, adding that an investigation will be carried out.
There was no immediate indication that the armed Islamist movement was responsible for the crash, though there has been an increase in Boko Haram activity in the state in recent weeks.
Boko Haram has reportedly taken over more than two dozen towns in Adamawa, Borno and Yobe states, including the commercial hub of Mubi, some 200km from Yola.
Last week, the extremists, who have been waging a five-year insurgency to create a hardline Islamic state, renamed Mubi "Madinatul Islam" or "City of Islam" in Arabic, residents said.
Nigeria's chief of army staff, Major General Kenneth Minimah, told a Senate defence committee on Thursday that the loss of territory was "painful" but promised that troops would recapture lost ground.
Locals and a government official said later that about 200 vigilantes and hunters armed with home-made guns, spears, clubs, bows and arrows, and machetes took back Mubi.
"It is true Mubi has fallen back into the hands of Nigerian soldiers with the help of local vigilantes and hunters," Chibado Bobi, chief of staff in the Adamawa state governor's office, told AFP.
"It is however too early for residents who fled to move back to Mubi because the security and vigilantes need to mop up all remnants of the group that may be lurking in nearby areas."
Boko Haram had introduced its strict version of Islamic law in the town, including amputations for accused thieves, according to residents who fled.Fear of attack
The hunters captured the Boko Haram-appointed emir, or leader, of Mubi after the attack, while militant fighters fled.
But instead of pulling back to other territory it is reported to control, locals said Boko Haram fighters invaded Hong, 50km south on the way towards Yola, and Gombi, to the northwest of Mubi.
In both Hong and Gombi, the militants were said to have razed the towns' police stations.
The move towards Yola will raise concerns about safety in the city, where thousands of people have taken refuge to escape the violence.
Thursday's chopper crash near a hall of residence at the Modibbo Adama University of Science and Technology caused panic among students, as weapons said to be on board apparently exploded.
"We heard a huge thundering sound which made us rush out of the hostels, thinking the school was under Boko Haram attack," said one student, Harisu Abdulaziz.
Another student said there was chaos as residents at the hostel tried to flee but were prevented by soldiers guarding the gates.
The university has been under military protection after a spate of Islamist attacks against schools in the region.
Kyari Mohammed, a lecturer at the university and a specialist in Boko Haram, said the downed helicopter was one of two that flew over the campus.
"The crash was followed by a series of intermittent explosions inside the chopper, which people believed to be from weapons the chopper was ferrying," he said.
On Monday, the military said another helicopter made an emergency landing near Yola airport. No casualties were reported.
The International Institute for Security Studies assesses that Nigeria's air force has limited combat capability because of an ageing fleet and funding issues.
This week, the United States said it had refused to transfer some Cobra helicopters to Nigeria because of concerns about the military's ability to use them.