WASHINGTON, Sept 29 (Reuters) - The U.S. Army will not be able to train for the full spectrum of military threats facing the United States before 2010 at the earliest because of the wars in Iraq and Afghanistan, the force's top civilian said on Monday.

Army Secretary Pete Geren said the 12-month period soldiers spend at home between deployments to the two counterinsurgency wars is not long enough to practice all the skills for conventional warfare against standing armies.

"We are out of balance right now," Geren said at a forum hosted by the Washington-based Center for Strategic and International Studies.

"We expect by 2010, 2011, we'll be in a position to expand the redeployment period, the reintegration period, and be able to start training across the full spectrum."

An Army official said the service hoped to extend so-called dwell times -- the period at home between deployments -- by expanding the active duty force to 547,000 troops by 2013 and by withdrawing additional forces from Iraq.

The Army, the largest of the U.S. armed forces, has about 140,000 troops deployed in combat zones overseas, Geren said. That compares with a total of 183,000 U.S. military personnel in Iraq and Afghanistan.

In the meantime, Geren said the U.S. Air Force or Navy could be called upon in an emergency to carry out operations that would normally fall to the Army.

"Lest an enemy think this provides an opportunity for them, we have other capabilities in our national arsenal," he said.

"If we were challenged, some missions that might have been given to the Army would be given to the Air Force and Navy. They would perhaps do them differently but they certainly have the power to deliver lethal force on any enemy."

He also said the United States, which has been frustrated by the unwillingness of some NATO countries to add troops in Afghanistan, should not expect allies to assume much of the burden in the global war on terrorism.

"Realistically, you look at who our friends are. Their populations aren't growing. Their defense budgets aren't growing. Their appetite for international engagements is modest relative to the threat," he said.

"We shouldn't fool ourselves into thinking that we're ever going to be able to shift much of this burden to others, as we'd like. I just don't think it's realistic." (Reporting by David Morgan; Editing by Andrew Gray and Cynthia Osterman)

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