WASHINGTON — The Pentagon yesterday announced upcoming deployments of more than 42,000 troops, including 25,000 active duty Army soldiers who would be sent to Iraq beginning in the fall to replace troops scheduled to come home by year’s end.

The deployments would maintain a level of 15 brigades in Iraq, or roughly 140,000 troops — the number military leaders expect will remain on the warfront at the end of July, once the currently planned withdrawals are finished.

Under the new Pentagon policy effective in August, those active duty Army units will serve for 12 months, rather than the 15-month tours that units in Iraq now are serving. The bulk of the soldiers deploying later this year returned from Iraq late last year, and will have gotten about a year at home to rest and retrain.

As part of the announcement, The Pentagon alerted four National Guard Army brigades, or about 14,000 troops, to begin preparing for deployments to Iraq beginning next spring, and one National Guard Army brigade, with about 3,100 soldiers, to prepare to deploy to Afghanistan in the spring of 2010.

The Guard announcements, a Pentagon spokesman, Bryan Whitman, said, are being made in advance so soldiers and their families can begin training and other preparations for their service.

Guard brigades heading to Iraq will provide security, while the brigade scheduled to go to Afghanistan in 2010 would train Afghan national forces.

The top American commander in Iraq, General David Petraeus, has said America will complete the withdrawal of the 20,000 troops that were sent to Iraq last year to tamp down the escalating violence in Baghdad. At the peak, there were 20 brigades with more than 170,000 American troops in Iraq.

Beyond that, he said he wants 45 days to evaluate the security conditions in Iraq, after which he will begin to decide whether more troops can be pulled out. The plan leaves open the possibility that America could keep 15 brigades there through the end of the year .