Nigerian forces used jets and attack helicopters to bombard Islamist militant camps in the northeast on Friday, killing a number of insurgents, the defense headquarters spokesman said.

Brigadier-General Chris Olukolade told Reuters by telephone several camps had been attacked, including in the Sambisa game reserve in Borno state, but did not have further details.

"A number of insurgents have been killed. It is not just Sambisa, every camp is under attack. But we have not done the mopping up operations on the ground to determine the numbers killed," Olukolade said.

Nigerian forces are trying to regain territory controlled by increasingly well-armed Boko Haram Islamist insurgents in their northeastern stronghold states of Borno, Yobe and Adamawa, which were put under a state of emergency by President Goodluck Jonathan on Wednesday.

Boko Haram, other Islamist militant groups such as al-Qaeda linked Ansaru and associated criminal gangs have become the biggest threat to stability in Africa's top oil producer.

Thousands have been killed since Boko Haram launched an uprising almost four years ago in an effort to create an Islamic state in a country of about 170 million split roughly equally between Christians, who are the majority in the south, and Muslims, who predominate in the north.

Violence has mostly happened far from the commercial hub Lagos or political capital Abuja and hundreds of miles away from oilfields in the southeast.

Source: Reuters