US troops will now serve up to 15 months in Iraq and Afghanistan instead of the usual 12-month tours under new defence department rules.
Soldiers on duty in Iraq will have to stay longer than expected
The move is designed to help the US military supply enough troops for ongoing operations.
Soldiers will be allowed a minimum of 12 months at home bases upon return.
The extended tours apply to troops currently in Iraq and those about to be deployed, Defence Secretary Robert Gates said.
The BBC's Adam Brookes, in Washington, says the announcement is further evidence that the US military is having to change the way it does business in order to meet its commitments in Iraq and Afghanistan and elsewhere.
Mr Gates' primary concern, however, is to ensure that in the long term, these strains do not "break" the US army, our correspondent says.
By guaranteeing 12-month periods of rest and re-training between tours in Iraq, and by increasing the size of the US army and marines, it seems Mr Gates hopes to avoid much more serious problems a few years down the line, our correspondent adds.
There are currently 145,000 US troops in Iraq. Reinforcements are needed to help the so-called "surge" against the insurgency that continues to deal heavy blows to Iraqi civilians and coalition forces.
"I think that what this recognises, though, is that our forces are stretched," Mr Gates said.
"There is no question about that... What we are trying to do here is to provide some long-term predictability for the soldiers and their families".
US Marines - part of the US Navy - are not subject to the extended tours and will serve a seven-month tour overseas and six months at home, in rotation. The Army National Guard and Army Reserve will continue to serve 12-month tours.
The Pentagon has been struggling to meet its own guidelines regarding the time troops get to spend at home.
Mr Gates said the new policy would remain in place until the defence department was confident enough to drop it back to 12-month tours, with 12 months at home and ultimately "to the rotation goal for army active duty forces of 12 months deployed and 24 months at home".
Chairman of the Senate Foreign Relations Committee, Democrat Joseph Biden said the decision was "an urgent warning that the administration's Iraq policy cannot be sustained without doing terrible long-term damage to our military".
The senator warned that the military could be forced to fully mobilise the National Guard and Reserve - perpetuating "a backdoor draft".
Meanwhile, the Bush administration has also confirmed that it is looking to appoint a senior official to oversee war and reconstruction efforts in Iraq and Afghanistan.
The Washington Post claims three senior generals have already refused to take the job that is being described as "war tsar".
Such a post is a new approach to managing Iraq and Afghanistan. Currently, a deputy National Security advisor oversees military policy in both conflicts, but does not directly report to the president.
The BBC's Daniel Lak, in Washington, says President Bush is known to feel civilian and military bureaucracies are not co-ordinating their efforts.