The units are the 42nd Infantry Division headquarters from the New York National Guard, the 256th Infantry Brigade from Louisiana, the 116th Cavalry Brigade from Idaho and Oregon, and the 278th Armored Cavalry Regiment from Tennessee, according to several officials who spoke on condition of anonymity.
The alert notifications were expected to be publicly announced later Monday at the Pentagon.
The exact number of Guard members who would be mobilized is unclear; the number could change depending on the security situation in Iraq during the course of this year, but they likely would total several thousand or more.
The four units have not been formally mobilized; those orders likely would come in several weeks.
The Pentagon is relying heavily upon Guard and Reserve troops in Iraq. Three Guard brigades — from Arkansas, North Carolina and Washington state — are part of the current troop rotation, which is in midcourse. They will spend a full year in Iraq, to be replaced by the newly alerted Guard units, if the Pentagon's current projection of troop requirements remains steady.
The troop rotation now under way is substituting about 110,000 active duty and Guard troops for the approximately 130,000 who have been in Iraq for a full year. The subsequent rotation, which is scheduled to take place roughly one year from now, is likely to involve about 100,000 more troops.
The active-duty units tapped for the 2005 rotation have not been publicly identified.
In other developments:
The new constitution, a key step in the U.S. plan to turn over power on June 30, will be signed by top American administrator L. Paul Bremer on Wednesday, after the Shiite Muslim religious holiday of Ashoura ends, a coalition official said on condition of anonymity. The charter will remain in effect until a permanent constitution is drafted and ratified next year.
The coalition official said the interim constitution strikes a balance between the role of Islam and the bill of individual rights and democratic principles, by calling Islam a source, but not a primary source, for the implementation of civil law.
The document does not resolve how power will be transferred from the U.S.-led occupiers to an Iraqi government by July.
But the charter stipulates that elections for a Transitional National Assembly, which will be charged with drafting and voting on a permanent constitution for Iraq, will be held by the end of the year, if possible. If not, those elections would be held in 2005.
Rules for those elections will be written by the interim Iraqi government that takes power July 1.
An Iraqi official said the politicians agreed on compromises on the role of Islam but put off details on some aspects of Kurdish autonomy.
Shiites, who dominate southern Iraq, insisted that if the Kurds had the right to self-rule in their northern strongholds, Shiites should enjoy the same privilege in areas of the south where they predominate.
The document was cobbled together in the early morning after two days of talks by members of the Governing Council and describes the future Iraq as a federalist state along the lines of Canada, Brazil and India, with considerable authority handed to individual regions.
The compromise also set a goal, not a quota, to have at least 25 percent of the national assembly made up of women.
With approval of the interim constitution, the last remaining step before the June 30 power transfer is to decide how to constitute a new government. The American blueprint called for choosing a legislature through regional caucuses, but the plan fell apart after Shiite clerics demanded the lawmakers be chosen in a national election.
The leading cleric, Ayatollah Ali al-Husseini al-Sistani, agreed to accept an unelected government to take power June 30 after the United Nations determined that an early vote was impossible. But Iraqi and U.S. officials have not agreed on a new formula and may require U.N. assistance to come up with one.
The deal came two days after a deadline set by the Americans and agreed to by the Governing Council in November. When it passed with the council still deeply divided, Bremer helped organize marathon talks.