United States Marines and Afghan forces have killed more than 40 suspected militants in the past week in an operation to take a remote mountain valley from insurgents who had inflicted the deadliest blow on American forces since the Taliban regime was ousted nearly four years ago, the US military said on Monday.

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The operation, which concluded over the weekend, was aimed at rebels believed responsible for twin attacks, including the downing of a helicopter, that killed 19 US troops in late June in Korengal Valley, eastern Kunar province.

"It was successful," Lt Col Jerry O'Hara told The Associated Press. "We had over 29 separate engagements with enemy forces that resulted in over 40 enemy killed in action and many others wounded."

He declined to give other details of the offensive. But O'Hara also announced that a separate three-day battle from Aug 7-10 in southern Zabul province's Daychopan district left a total of 65 suspected militants dead. The military had previously reported that 16 rebels had been killed in the fighting.

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News of the casualties comes after a deadly period for US forces in Afghanistan, with 13 American troops killed this month. Four soldiers were killed Sunday when a massive bomb exploded under a wooden bridge as a convoy of armoured Humvees was crossing it. Shrapnel from secondary explosions wounded three troops as they tried to pull the four out of a burning Humvee.