WASHINGTON - As security improves in Iraq, pressure is building to reverse one of the most onerous decisions Defense Secretary Robert M. Gates made to enable President Bush's troop buildup to go forward this year: extending the tours of active-duty soldiers from 12 months to 15 months.

The extra three months is a weighty burden physically and psychologically for soldiers already stressed by multiple tours and families coping with strains that have mounted since the war began in 2003.

"We can't sustain that," Gen. George Casey, who was the top U.S. commander in Iraq before becoming the Army chief of staff at the Pentagon in April, said recently. "We have to come off that." He said a decision on cutting tour lengths could be announced in three or four months.

Army leaders are pushing to shorten tour lengths to 12 months by summer, when Bush's troop buildup is scheduled to end. But senior commanders in Baghdad appear reluctant to agree to a change until perhaps late next year, fearing that Iraqi stability still will be in doubt until then.

The outcome depends in large part on what Bush decides to do next spring after hearing an updated assessment of Iraq from his top commander in the country, Gen. David Petraeus. At hand then will be a decision on whether to continue cutting U.S. troop levels beyond July. If no further cuts are made, it will be much harder for the Army to back away from the 15-month tours.

There are now 166,000 U.S. troops in Iraq, about 30,000 more than when Bush announced his buildup last January. By July that is supposed to have fallen back to about 135,000. Although Bush has not committed yet to going lower, Gates has expressed hope deployment could drop to 100,000 by next December.

The 15-month standard does not apply to Army National Guard or Army Reserve soldiers. Marines generally serve seven-month tours, although they get less time between tours than do soldiers.

Lt. Gen. Ray Odierno, the No. 2 U.S. commander in Iraq, said in a recent interview that his calculation about how many Army brigades were needed and for what periods were not based on the policy goal of getting back to 12-month tours, even though it is a goal he supports. Instead he takes into consideration the trends in security gains, the fighting capacity of the Iraqi army, and the progress in Iraqi governance.

Casey, as the Army chief of staff, mentioned that he had thought about the danger of allowing wartime strains to "break" the Army.

"There's a thin red line out there that you don't know when you cross it until after you've crossed it," Casey said. "We are now in a position of having to sustain an all-volunteer force in a protracted confrontation for the first time since the Revolutionary War. . . . I've got to tell you, it's a dicey game."