Nigeria's President Goodluck Jonathan on Tuesday declared his bid for re-election, vowing to finally defeat Boko Haram whose rise in strength during his first term has threatened the country's sovereignty.

The 56-year-old made the announcement to tens of thousands of supporters in the red, white and green of his ruling Peoples Democratic Party (PDP), at a carefully orchestrated ceremony including patriotic music, dancing, prayers and speeches.

Islamists from Nigeria's Boko Haram killed at least three civilians in six "coordinated attacks" in the remote north of neighboring Cameroon, a defense ministry source told AFP on Tuesday.

"Six attacks were carried out almost simultaneously on Sunday," the source said on condition of anonymity, saying the fighters were pushed back across the border by Cameroonian troops.

The Nigeria military said one of its helicopters made an emergency landing in the restive northeast on Monday, while witnesses and a security source reported that the chopper crashed with several foreigners on board.

Defense spokesman Chris Olukolade said the helicopter left on a training mission at about 9:30 am (0830 GMT) from the airport in Yola, capital of Adamawa state, which is under a state of emergency because of Boko Haram's violent uprising.

Boko Haram was suspected of killing nearly 50 pupils in a suicide bombing in northeastern Nigeria on Monday, in one of the worst attacks against schools teaching a so-called Western curriculum.

The explosion ripped through an all-boys school in Potiskum just as students gathered for morning assembly before classes began, causing panic and chaos.

Nigeria's military on Saturday released 125 people it said were arrested in an operation against Boko Haram but were later found to not be part of the Islamist group. 

The latest release came two days after 42 others were set free in similar circumstances. 

A bomb concealed in a black plastic bag killed several people in Nigeria's restive northeast on Friday, as a regional governor warned that more troops were needed to stem a sweeping advance by Boko Haram.

The fresh violence in Bauchi state came days after Islamist fighters rampaged through two towns in neighboring Gombe, overpowering the military at a checkpoint and raiding a factory owned by the French cement giant Lafarge.

Boko Haram has taken over at least five municipalities in northeast Nigeria's Adamawa state, its governor said on Friday, calling for more troops to halt further Islamist gains.

"I can talk of my entire (home) district... Five local governments have been overrun," Governor Bala Ngilari told journalists on a visit to the capital, Abuja.

Sixteen men who were arrested by Nigerian soldiers in the country's northeast were found dead just hours later with bullet wounds, community leaders said on Thursday, demanding an inquiry.

Troops rounded up 17 people, including an imam, from the Dogo Tebo area of Potiskum in Yobe state as they left a mosque after morning prayers on Wednesday.

Boko Haram has renamed two towns it captured in northeast Nigeria as part of campaign to establish a caliphate in the region, residents who have fled in recent days told AFP on Thursday.

The Islamists seized the town of Mubi in Adamawa state last week and now insist it be called Madinatul Islam, or "City of Islam" in Arabic.

Sixteen men who were arrested by Nigerian soldiers in the country's northeast were found dead just hours later with bullet wounds, community leaders said on Thursday, demanding an inquiry.

Troops rounded up 17 people, including an imam, from the Dogo Tebo area of Potiskum in Yobe state as they left a mosque after morning prayers on Wednesday.