Nigerian forces have launched attack on Boko Haram strongholds in the country’s northeast region, security sources have told Reuters news agency, amid deployment of fighter jets and helicopter gunships.

Soldiers raided areas in the Sambisa Game Reserve in Borno state where the armed group has established bases, two sources, who spoke on condition of anonymity, said on Thursday without giving details.

Nigeria's military said that it was ready to launch air strikes against Boko Haram fighters as several thousand troops moved to the remote region to retake territory seized by the group.

A force of "several thousand" soldiers along with fighter jets and helicopter gunships have been deployed for the offensives in Borno, Yobe and Adamawa state, according to defence spokesman Brigadier General Chris Olukolade.

Telephone connections to Borno and Yobe were almost completely cut on Thursday, and a 12-hour overnight curfew has been imposed in Adamawa, following the other two states which are already under curfew.

The operation comes after President Goodluck Jonathan imposed a state of emergency in all three areas as he admitted that Boko Haram had "taken over" territory in the northeast and declared war against the government.

Upsurge in violence

The increased military presence follows an upsurge in violence against government and Christian targets in the northeast by Boko Haram, who want an Islamic state in Nigeria's north. 

The offensive has been cautiously welcomed by some in Nigeria, but the army's reputation for excessive force is causing concern around the world.

Rights groups said they feared for the safety of civilians from combatants on both sides, but Jonathan's move enjoys widespread public support after more than three years of trying to contain the insurgency without notable success.

The United States expressed fears over a worsening "cycle of violence" on Wednesday, and warned that any "heavy-handed" tactics or disregard for human rights during the emergency operations could damage bilateral relations.

Rights groups have documented cases of abuses by Nigerian forces, including summary executions and random shootings.

"If the military continues its practice of targeting civilians, there is a risk of massive abuses during this offensive," Eric Guttschuss, from Human Rights Watch, said.

Source: Agencies