Updated

Four major Army National Guard (search) units have been placed on alert for possible deployment to Iraq (search) late this year or in early 2005 as part of a larger force rotation, officials said Monday.

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 alerts were issued well in advance in order to give the Guard members adequate time to prepare for the likelihood of being mobilized and sent to Iraq for 12-month tours. Many Guardsmen and some members of Congress complained that earlier mobilizations for Iraq came with little advance notice.

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.