WASHINGTON -- The U.S. military may not be able to win any new wars as quickly as planned because the conflicts in Iraq and Afghanistan have strained its manpower and resources, the nation's top military officer told Congress in a classified report.

Gen. Richard Myers, chairman of the Joint Chiefs of Staff, described the U.S. military as in a period of increased risk, according to a senior defense official, who described the report Tuesday on the condition of anonymity.

"We will prevail," Myers said when asked about the report. "The timelines [to winning a new war] may have to be extended and we may have to use additional resources, but that doesn't matter because we're going to be successful in the end."

Myers predicted the risk would go down in a year or two, the official said. Myers provided the report to Congress on Monday.

Still, the report says the U.S. military is able to win any conflict it becomes involved in, said Pentagon spokesman Bryan Whitman.

"We are at war and that level of operations does have some impact on troops," White House spokesman Trent Duffy said. "But the president continues to be confident, as well as his military commanders, that we can meet any threat decisively."

The military's reorganization toward Defense Secretary Donald Rumsfeld's vision of a lean, agile force, should reduce what increased risk it is facing, Whitman said.

Among the most likely conflicts the Pentagon foresees in the near term are with North Korea and Iran, the two remaining members of President Bush's "axis of evil." The Bush administration accuses both of having ambitions to become a nuclear power; North Korea has already claimed it has nuclear weapons.

The U.S. military has timelines in place for defeating its potential adversaries, given enough soldiers, tanks, aircraft and warships to do the job. But with so much of those resources tied up fighting insurgencies in Iraq and Afghanistan, those timelines could slip, Myers said, according to the defense official.

About 138,000 American troops are in Iraq, according to U.S. Central Command. Another 18,000 are in Afghanistan.

Military officials have given no precise estimate when they will be able to significantly draw down the number of U.S. troops in Iraq, but some generals have suggested it could come next year if Iraqi security forces continue to improve in quality and grow in numbers.