Boko Haram crisis: Nigerian army begins raids in Borno
Nigeria's army has begun operations against militant Islamists in the north-east, military officials say.
They say troops raided parts of a game reserve in Borno state where the Boko Haram group has established bases.
The raids came after states of emergency were declared in Borno, Yobe and Adamawa north-eastern states, where some 2,000 people have died since Boko Haram launched an insurgency in 2009.
Meanwhile, explosions and gunfire have been heard overnight in Katsina state.
Residents have told BBC Hausa that banks, police stations and prisons were destroyed in the town of Daura, near the border with Niger.
They said they had seen the bodies of five security agents and three militants, but there has been no official confirmation of casualties.
Mobile phone networks were not functioning in many parts of north-east Nigeria on Thursday, but the BBC's Will Ross in Abuja says it is not clear if this is related to the current military offensive.
A security official told the AP news agency that the mobile phone service had been shut down during the military operation.
Militants have previously attacked mobile phone masts in the area in an effort to disrupt communications.
On Thursday soldiers raided "terrorist camps" in the Sambisa Game Reserve, a 500 sq km (200 sq mile) savannah in Borno that is known to be a haven for Boko Haram militants, officials were quoted as saying.
In January, the military said it had deployed helicopter gunships to destroy Boko Haram camps in the same park, which is about 70km (45 miles) south of the Borno state capital, Maiduguri, where the militants first emerged in 2009.
The unnamed security official told AP that 21 people had been killed when the camps were shelled.
Nigerian military spokesman Brig Gen Chris Olukolade said "several thousand" troops had been sent to the three north-eastern states to tackle Boko Haram.
He told the AFP news agency that the "entire Nigerian military is involved in this operation, including the air force".
"Air strikes will be used when necessary," he said.
The three semi-desert states, which border Niger, Chad and Cameroon, are roughly the size of England or Illinois but have a population of only 10 million.
Will Ross says targeting Boko Haram's rural bases or training camps should pose no great challenge for the military; the hardest part of this campaign will be in urban areas like Maiduguri, where the militants are living among the civilian population.
On Thursday, a dusk-to-dawn curfew was imposed in Adamawa to curb militant attacks.
The BBC's Abdullahi Tasiu Abubakar in Adamawa city says many there feel the curfew is unnecessary, as the security situation there is less serious than in Borno and Yobe.
Our reporter adds that there is no sign of a huge military build-up in the city since President Goodluck Jonathan declared a state of emergency on Tuesday in the three states.
The president said the army would take "all necessary action" to "put an end to the impunity of insurgents and terrorists".
Boko Haram, whose name means "Western education is forbidden" in the local Hausa language, is fighting to overthrow the government and create an Islamic state in the north.
Although they often attack Christians and government targets, they have also killed many Muslim civilians.