By Lanre Ola and Imma Ande
MAIDUGURI/YOLA, Nigeria (Reuters) - Nigerian forces attacked Islamist strongholds in the northeast on Thursday, security sources said as an offensive got under way to wrest back territory from increasingly well-armed Boko Haram insurgents.
Soldiers raided areas in the Sambisa Game Reserve, a remote savannah of some 500 sq km (200 sq miles) in Borno state where Islamists have established bases, said two sources who spoke on condition of anonymity. They gave no further details.
Preparing for possible further action across three frontier states where President Goodluck Jonathan declared a state of emergency on Tuesday, the armed forces also deployed jet fighters and helicopter gunships to the region.
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.
It follows an upsurge in violence against government and Christian targets in the northeast by Islamists who want an Islamic state in Nigeria. Africa's most populous nation's 170 million people are split evenly between Christians, who dominate in the south, and Muslims, who are the majority in the north.
Little detail was available from Sambisa. Nigerian forces have attacked Islamist bases in the area of the game reserve before, as recently as February, to rout militants seen as the biggest security threat to Africa's top energy producer.
The emergency affects the semi-desert states of Borno, Yobe and Adamawa, which variously border Niger, Chad and Cameroon and cover some 150,000 sq km (60,000 sq miles) - an area similar to England or Illinois, but with a population of only 10 million.
A Reuters reporter saw two Alpha light attack jets land at Yola in Adamawa state. Air Force spokesman Air Commodore Yusuf Anas confirmed that "air assets", also including helicopter gunships, had been sent to support ground troops. A military source said there could be air strikes on Islamist bases.
In the 1980s, military leaders used air power to put down religiously inspired protests during a crackdown that left some 5,000 people dead, according to state media at the time.
Telephone connections to Borno and Yobe were almost completely cut on Thursday. In Adamawa, where a new, 12-hour overnight curfew was declared - the other two states were already under curfew - some cautiously welcomed the offensive.
"This state has been under the control of gunmen for so long, it's been long overdue," said Audu John, a market trader.
But another man, Ahmed Usman, feared civilians would become targets for killings or torture by a military notorious for abuses. His family was evacuating as soon as possible, he said.
The Islamist insurgency has cost thousands of lives since it began in 2009, when a crackdown killed 800 people, including Boko Haram founder Mohammed Yusuf, who died in police custody.
Because it has mostly happened far from economic centers such as the commercial hub Lagos or political capital Abuja - and because it is hundreds of miles away from oilfields in the southeast - it has not been a priority for the establishment.
The offensive ordered by Jonathan, a southern Christian, may answer critics who had accused him of failing to address the crisis: "The federal government has come to terms with the bleak reality that what we are facing is ... terrorism in its most horrific form," the Punch newspaper said in an editorial.
"Nigeria is teetering on the precipice of disintegration.
"It is time to act decisively."
But the United States expressed concern about a worsening "cycle of violence" on Wednesday, a view echoed by human rights groups Amnesty International and Human Rights Watch on Thursday.
Both have documented cases of abuses by Nigerian forces, including summary executions and random shootings.
At Human Rights Watch, Eric Guttschuss said: "If the military continues its practice of targeting civilians, there is a risk of massive abuses during this offensive."
(Additional reporting by Pascal Fletcher in Johannesburg, Tim Cocks in Lagos, Joe Brock in Abuja and Ibrahim Mshelizza in Maiduguri; Writing by Tim Cocks; Editing by Philippa Fletcher and Alastair Macdonald)