WASHINGTON — President Bush plans to announce on Thursday that Army units heading to war after Aug. 1 will serve 12-month tours rather than the 15 months that soldiers are currently deployed, senior defense officials said.

The reduced tour length will not apply to any soldiers now serving in Iraq, Afghanistan or other war zones, although that could change later if security conditions improve, the officials said Wednesday. They spoke on condition of anonymity in advance of Bush's speech.

Bush planned to make the announcement during a morning speech, which is expected to include his endorsement of an indefinite pause in troop cuts after July to reassess force levels in Iraq.

There are 160,000 troops in Iraq, including about 18 combat brigades, and that number is expected to go down to 140,000 — including 15 brigades — by the end of July. Military leaders have consistently said that 15-month tours put too much stress on soldiers and their families.

After Bush's speech, Defense Secretary Robert Gates and Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, were expected to answer questions about the shorter tours when they testify Thursday before the Senate Armed Services Committee.

Under the new plan, soldiers would deploy for one year, then get 12 months at home to rest and then retrain for their next tour.

The shorter deployments will apply broadly to Army units. But officials acknowledge that there could be some individuals or smaller specialized companies with jobs in high demand who could be pressed into longer service.

Gates and Mullen are also expected to speak more broadly about their desire to send additional U.S. forces to Afghanistan at some point next year. But they were expected to note that the timing and size of that Afghanistan buildup will depend on the situation in Iraq, and whether they have been able to continue to reduce the number of troops in Iraq.

The Bush announcement only applies to active-duty Army soldiers. National Guard and Reserve units are already limited to 12-month tours, and Air Force, Navy and Marine units routinely serve shorter deployments.