The orders had been anticipated, but the specific units were not announced until Monday. They are the Army National Guard's 39th Infantry Brigade Combat Team, from Arkansas; the 45th Infantry Brigade Combat Team, Oklahoma; the 76th Infantry Brigade Combat Team, Indiana; and the 37th Infantry Brigade Combat Team, Ohio.

The units would serve as replacement forces in the regular troop rotation for the war and would not be connected to the recent military buildup for security operations in Baghdad, the Defense Department said.

They are expected to deploy under rules of Defense Secretary Robert Gates' new policy of mobilizing reserves for no longer than one year at any one time.

One unit would deploy in December and the remaining in 2008, the Army said in a statement.

"They are receiving alert orders now in order to provide them the maximum time to complete their preparations," the Defense Department said in a separate statement. "It also provides a greater measure of predictability for family members and flexibility for employers to plan for military service of their employees."

The final determination whether the units will deploy will be made considering conditions on the ground in Iraq, officials said.

The troop alerts come as President George W. Bush and Congress wrestle over legislation that would set timelines for troop withdrawals from Iraq.

Bush asked for more than $100 billion (euro75 billion) to pay for the wars in Iraq and Afghanistan this year. Congress has approved the money, but the Senate added a provision urging most U.S. combat troops to be out of Iraq by March 31, 2008. The House version demands a September 2008 withdrawal. Bush has promised to veto any legislation that includes such deadlines.

Smaller units and individual troops from the Guard already have returned to Iraq, and some active duty units have served multiple tours. The Army said some of the troops being alerted now have not yet served in Iraq, but some have served in the campaign in Afghanistan or elsewhere.

"These outstanding citizen soldiers, their family members and employers continue to make great sacrifices for the good of our country, and they appreciate and acknowledge the continued support of the American people," the Army said.

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