If their assignment to Iraq is ultimately approved by Defense Secretary Robert Gates, it would be the first time full Guard combat brigades have been sent back to Iraq for second tours.
A brigade is roughly 3,500 troops.
The units would serve as replacement forces in the regular unit rotation for the war and would not be connected to the recent military buildup for security operations in Baghdad. Defense Secretary Robert Gates is expected to sign the notices alerting the Guard troops shortly, said the official, who requested anonymity because the information has not yet been released.
"You will start to see reserve component forces coming back into the rotation," said the official, adding that the notices are being done now in order to give the Guard units more time to prepare.
Guard officials told The Associated Press in February that they had contingency plans to send at least two Guard combat brigades back to Iraq in 2008 for their second yearlong tour of duty.
While it is not clear yet which units would be alerted, they probably would include brigades that were among the first to go to Iraq early in the war.
Smaller units and individual troops from the Guard already have returned to Iraq for longer periods, and some active duty units have served multiple tours.
The troop alerts come as President Bush and Congress wrestle over legislation that would set timelines for troop withdrawals from Iraq.
Nearly two months ago, Mr. Bush asked for more than $100 billion to pay for the wars in Iraq and Afghanistan this year. Congress has approved the money, but the Senate added a nonbinding provision calling for most U.S. combat troops to be out of Iraq by March 31, 2008. The House version demands a September 2008 withdrawal. President Bush has vowed to veto any legislation that includes such deadlines.
According to defense and Guard officials, the first Guard units could go as early as late December with others to follow during the next six months. They would be deployed only if commanders in Iraq determined the troops were needed.
About 270,000 of the more than 347,000 Army Guard soldiers have served in the Afghanistan and Iraq wars.
Gates said Thursday that the Pentagon's goal is to give reserve units five years at home for every year deployed. Earlier this year he announced that the reserves will now deploy as full units, and they will go for 12 months at a time.
Guard units currently serve about 18 months for each tour of duty, including six months of training.
But Gates told Pentagon reporters that there will be a "transition period during which those guidelines would be violated and in which we would be unable, because of the troops commitments in Afghanistan and in Iraq, to meet those goals."
That transition period, he said, could last a year or two.