If Bush follows the recommendations, he would delay any additional buildup in Afghanistan until early next year, when another brigade would be deployed there instead of to Iraq.
That move would cut the number of brigades in Iraq to 14 in February.
The plan is aimed at taking advantage of security gains in Iraq to bolster the military effort in Afghanistan, where violence is on the rise. Several senior military and defense officials described the recommendations on condition of anonymity because the plan has not been made public.
They also acknowledged the plan is a compromise since Gen. David Petraeus, the top U.S. commander in Iraq, argued to maintain the current force levels in Iraq - about 146,000 troops, including 15 combat brigades and thousands of support forces - through June.
Bush is weighing the recommendations; in the past, he has largely accepted the military's advice. If he adopts them, it would be left to the next president to execute further troop reductions in Iraq and a greater buildup in Afghanistan. Bush's term ends in January.
Democratic presidential nominee Barack Obama has advocated pulling all U.S. combat forces out of Iraqi within 16 months of taking office. GOP nominee John McCain has said he would rely on the advice of U.S. military commanders to determine the timing and pace of troop reductions. Both candidates have said more troops are needed in Afghanistan.
It had been widely expected that Petraeus would recommend a faster pullback in Iraq, perhaps reducing the number of combat brigades from 15 to 14 this fall. But several recent events may have changed the calculus.
Among the more important changes was the unanticipated decision by Georgia to bring home its contingent of about 2,000 soldiers after Russia invaded the former Soviet republic in early August.
Also arguing in favor of a smaller reduction this fall was the inability of the Iraqi government to move ahead with provincial elections in October as originally planned. No firm date for the balloting has been set, but it is generally believed that the long-anticipated elections will not happen before December.
At the same time, however, military leaders have become increasingly concerned about escalating violence in Afghanistan, and they don't want to sit idle as the winter approaches, giving the enemy more time to build its forces.
One senior military official said it was considered critical to replace the Marines in Afghanistan beginning this year.
"We believe the risk in Afghanistan is such that we need to do something, and the risk in Iraq is such that we can go into Afghanistan without risking unduly the posture in Iraq," said the official.
Pentagon officials believe the greatest challenge is to identify enough support troops to provide essential logistics and intelligence assets for the additional U.S. units heading to Afghanistan.
Without that support - which includes the delivery of weapons and food and the construction of roads and runways - the Pentagon's plan would require a significant increase in military facilities in Afghanistan, including forward operating bases, like those in Iraq.
Pentagon leaders have struggled to balance the two warfronts, repeatedly stressing that Iraq is the priority.
On several occasions, Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, has said that, "In Afghanistan, we do what we can; in Iraq, we do what we must."
But a resurgence of the Taliban in Afghanistan, coupled with the improved security in Iraq has forced a greater emphasis on Afghanistan.
Violence has plunged in Iraq's western Anbar province, which until early last year was a stronghold for the insurgency. That will allow a battalion of Marines - or roughly 1,000 - to go to Afghanistan to train security forces in November rather than going to Iraq as initially planned.
They would replace a Marine unit currently training Afghan security forces, but a second Marine unit now doing combat operations would not be replaced until early 2009, probably by an Army brigade.
There has been speculation that the 3rd Brigade, 10th Mountain Division, which is slated to go to Iraq, will instead go to Afghanistan. That unit, which is based at Fort Drum, N.Y., has previously served in Afghanistan.
Military leaders have insisted in recent months that over time they need to beef up forces in Afghanistan by as many as 10,000 troops - the equivalent of about three combat brigades.