President Obama announced last night that he was sending up to 17,000 extra US troops to Afghanistan, but he has not yet decided how many more to deploy there to confront the Taleban and al-Qaeda insurgency.

“This increase is necessary to stabilise a deteriorating situation in Afghanistan, which has not received the strategic attention, direction and resources it urgently requires,” Mr Obama said in a statement.

Many of the new troops had been scheduled to head to Iraq, but have been “re-missioned” to Afghanistan to tackle what Mr Obama calls his top foreign policy priority.

The reinforcement is significantly less than the 30,000 figure that has been under discussion for months. That could still be reached by the end of this year, but Mr Obama is being cautious in how many he immediately sends until military chiefs define the mission and an ultimate exit strategy.

A further 3,000 US troops have begun arriving in Afghanistan and have already seen combat in their mission of securing two violent provinces near the capital, Kabul. The new unit — the 3rd Brigade Combat Team of the 10th Mountain Division — moved into Logar and Wardak provinces last month.

The announcement of the long-awaited US “surge” came as it emerged that the number of civilians killed in Afghanistan rose by 40 per cent in 2008, according to a report released by the United Nations.

It said that 2,118 civilians died as a result of the conflict in Afghanistan last year — 55 per cent killed by insurgents, and 39 per cent by Western forces.

Of the 829 killed by Western troops, 522 died in airstrikes; an issue on which Hamid Karzai, the Afghan President, has repeatedly challenged Western commanders and politicians.

Nato forces killed 31 per cent more civilians than in 2007, in spite of assurances that greater care is being taken in the face of Afghan public anger over the rising casualties. Western military officials suggested that the rise was attributable in part to the intensification of violence across the country.

A general increase in violence across Afghanistan claimed a total of 8,600 lives, the UN said, with 38 aid workers killed, double the number in 2007. They included Gayle Williams, a British aid worker shot dead on a Kabul street in October 2008.

John McCain, the Republican Senator defeated by Mr Obama in the presidential election, endorsed the decision to send more troops, but called for a strategic overhaul.

“While I welcome today’s announcement, I hope it is just the first step in a new comprehensive approach to Afghanistan. A major change in course is long overdue,” he said.

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