The US Army is extending the tours of duty for soldiers serving in Iraq and Afghanistan by three months to allow soldiers at home enough time to recover and re-train between deployments.

Five-and-a-half years of fighting the War on Terror has forced the Pentagon to rejig its resources, with this year's surge of 22,000 extra soldiers for Iraq placing an added strain on the US Army, which has already cut the amount of time that units get to recuperate before being sent back into combat.

Robert Gates, the Defence Secretary, announced the extension of a standard tour of duty from 12 to 15 months last night, admitting: "Our forces are stretched; there’s no question about that."

The longer tours will affect about 100,000 American soldiers currently in Iraq and Afghanistan, and the thousands more who are preparing to deploy. It does not affect the Marine Corps or the National Guard or Reserve.

This year's build-up of troops in Iraq, ordered by President George Bush in January, has forced the Army to break its standard cycle of tours of duty followed by a year of re-training. By boosting the number of brigades in the country from 15 to 20, the Pentagon was faced with the choice of sending troops into combat after less than a year away from the front lines or to extend the tours of those already in theatre.

The extra burden for soldiers and their families will add to the political cost of a war that has been exposed to sustained and mounting criticism since the Democratic takeover of Congress on January 1. Last night's announcement comes shortly after National Guard brigades from four states were informed that they were needed in Iraq for a second tour of duty, drawing complaints from governors.

Last year the Pentagon spent an extra $1 billion (£508 million) on bonuses to maintain financial incentives for soldiers and the Marines.

Describing the extended tours as "a difficult but necessary interim step,” Mr Gates said that new policy would give the Pentagon the capability to maintain the higher force levels in Iraq until next April. He did not say how long the modified tours of duty would stay in place.

Marine General Peter Pace, the Chairman of the Joint Chiefs and America's most senior uniformed officer, acknowledged that the tours would make life harder for those on the ground in Iraq and Afghanistan.

“Is it an additional strain to go from 12 months to 15 months? Of course it is,” he said. “Is it in combat and therefore even more difficult? Of course it is. And that’s why the entire nation should be thankful that we have such incredible young men and women who, knowing that, who volunteer to serve this nation in a time of great need.”

Democratic politicians said the news would come as a blow to military families. “Extending the tours of all active-duty Army personnel is an unacceptable price for our troops and their families to pay,” said Nancy Pelosi, the Democratic Speaker of the House.

Representative Ike Skelton, chairman of the House Armed Services Committee, said the longer tours will have a “chilling effect" on the Army's current struggle to recruit and retain the soldiers it needs. “We also must not underestimate the enormous negative impact this will have on Army families,” he said.

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