President George W. Bush plans to announce a return to 12-month tours for US Army troops in Iraq and Afghanistan, ending 15-month tours that have strained the force, a US defense official said Wednesday.

The longer tours were imposed a year ago to enable the surge of US forces to Iraq, but are now expected to go back to 12 months in the fall after the last of the surge troops are out of the country.

Bush planned to announce the decision to return to shorter tours on Thursday, the official said, speaking on condition of anonymity.

The White House said he will give a speech on Iraq and US troop levels at 11:30 am (1530 GMT).