Britain will withdraw all of its combat troops from Afghanistan before the end of 2015, Prime Minister David Cameron promised on Saturday, making what he called a "firm commitment".

"The commitment we have entered into today to transfer lead responsibility for security to the Afghan government by the end of 2014 will pave the way for British troops to be out of Afghanistan by 2015," he said.

"This a firm deadline that we will meet," he told reporters at the NATO summit in Lisbon, staking out a tougher position on troop withdrawals than other NATO allies, who have not set a final cut-off date for operations.

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