WASHINGTON (AP) – Beginning immediately, all active-duty Army soldiers in Iraq and Afghanistan will serve 15-month tours – three months longer than the usual standard, Secretary of Defense Gates said Wednesday.

It was the latest move by the Pentagon to cope with the strains of fighting two wars simultaneously and maintaining a higher troop level in Iraq as part of President Bush’s revised strategy for stabilizing Baghdad.

“This policy is a difficult but necessary interim step,” Mr. Gates told a Pentagon news conference.

He said it did not affect the Marines, whose standard tour is seven months, nor the Army National Guard or Army Reserve, which will continue to serve 12-month tours.

Mr. Gates said the new policy also seeks to ensure that all active-duty Army units get at least 12 months at home between deployments. He said it would allow the Pentagon to maintain the current level of troops in Iraq for another year, although he added that there has been no decision on future troop levels.

Some units had already been extended beyond 12 months by varying amounts. The new policy will make deployments more equitable and more predictable for soldiers and for their families, Mr. Gates said.

“I think it is fair to all soldier that all share the burden equally,” he said.