Nigerian warplanes have struck suspected Boko Haram camps in the country's northeast as part of an ongoing military offensive against the armed group.

Troops used jets and helicopters on Friday to hit targets in their biggest military campaign since Boko Haram launched a revolt almost four years ago.

The air raids, which began after President Goodluck Jonathan declared a state of emergency in the states of Yobe, Borno and Adamawa, drew a sharp warning from the US, which called on the government to ensure rights are not violated.

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Boko Haram, an armed group that is fighting Western influence and wants to form an Islamic state, has staged a series of deadly attacks, mainly in the predominantly Muslim north.

A military source said at least 30 fighters had been killed in the fighting.

John Kerry, the US secretary of state, issued a statement saying "we are ... deeply concerned by credible allegations that Nigerian security forces are committing gross human rights violations".

The violations, he said, "will, in turn, only escalate the violence and fuel extremism".

Restraint urged

The US is the biggest foreign investor in Africa's most populous nation, notably in its energy sector, and buys a third of Nigeria's oil.

The US "condemns Boko Haram's campaign of terror in the strongest terms", Kerry said, urging Nigeria's armed forces to show restraint and discipline.


Rights groups have previously compiled reports blaming security forces and Boko Haram for rights abuses.

Brigadier-General Chris Olukolade, Nigerian defence spokesman, said in a statement that troops destroyed several Boko Haram camps and weapons stockpiles in forests around Borno state, epicentre of the uprising and relic of a medieval Islamic empire.

"Heavy weapons including anti-aircraft and anti-tank guns were also destroyed in the process," he said.

"The special operations ... resulted in the destruction of much of the insurgents' weapons and logistics such as vehicles, containers, fuel dumps and power generators."

He said the death toll among the fighters would be verified during mopping-up exercises in the camps, including in the Sambisa game reserve in Borno.

A military source said at least 30 fighters had been killed in one operation.

Boko Haram fighters, seen as the main security threat to Africa's biggest oil producer, have been staging bolder attacks since last month, including one on the town of Bama that left 55 dead.

Source: Agencies