WASHINGTON — The Pentagon is developing plans that for the first time would send entire National Guard combat brigades back to Iraq for a second tour, the Guard's top general said in the latest sign of how thin the military has been stretched.

Smaller units and individual troops from the Guard have already returned to Iraq for longer periods, and some active-duty units have served multiple tours. Brigades generally have about 3,500 troops.

The move — which could include brigades from North Carolina, Florida, Arkansas and Indiana — would force the Pentagon to make the first large-scale departure from its previous decision not to deploy reserves for more than a cumulative 24 months in Iraq.

For some units, a second tour would mean they would likely exceed that two-year maximum. The planning was described by Lt. Gen. H. Steven Blum, who commands the Guard, in an interview this week.

About 2,200 citizen soldiers of the Hawai'i National Guard and Army Reserve have served year-long deployments in Iraq, starting in July 2004. Most of the group transitioned out of Iraq and into Kuwait in 2005 and returned to the Islands.

In a related move, the Pentagon is preparing to release a list of active units — and perhaps reserves as well — scheduled to go to Iraq that would largely maintain the current level of forces there over the next two years, another senior defense official said on Thursday. There are about 145,000 U.S. troops in Iraq.

That official requested anonymity because the plan has not been made public.

The Pentagon routinely notifies units to prepare for deployment, knowing it is easier to cancel a move overseas than to suddenly make such a large troop movement.

It was not clear whether this week's resignation of Defense Secretary Donald Rumsfeld would affect deployment plans. President Bush has selected former CIA chief Robert Gates, who has criticized U.S. policy in Iraq, to replace Rumsfeld, but he has not yet been confirmed by the Senate.

"We are doing contingency planning for one or two (units), and we have contingency plans for more than two if necessary," Blum said on Wednesday. The North Carolina brigade, he said, is being considered since it was one of the first to go to Iraq after the war began in 2003.

Blum also said defense officials have been discussing whether they need to adjust their policy that limits the deployment of reserves in the war to 24 months.

"When that policy was originally formulated, I seriously doubt anyone thought we would be where we are today, at the level of commitment that is necessary today," he said.

Just last month, defense officials said the Marines are drawing up similar plans that would for the first time send some reserve combat battalions back to Iraq for a second tour.

Under the authority by which Bush ordered a call-up of the Guard and Reserve after the Sept. 11, 2001, terrorist attacks, troops could be mobilized an unlimited number of times as long as each mobilization is no longer than 24 consecutive months.

Until now, Pentagon officials have interpreted that as 24 cumulative months.

While the ultimate goal for the National Guard is to deploy one year overseas and spend six years at home, Blum said the current demands could force soldiers to deploy as often as one year every three or four years.

Blum said he believes that Guard combat brigades are prepared and willing to make a second trip to Iraq if needed.

Blum said the first units to deploy in the war — such as the 30th Infantry Brigade from North Carolina, the 76th Infantry Brigade from Indiana, the 53rd Infantry Brigade from Florida, and the 39th Infantry Brigade from Arkansas — would likely be among those first called for a second tour.

"Logic would lead you to go back to the ones that went first, and start going around again," said Blum. "But that's probably not exactly how we'll do it" because the decision will depend partly on what types of units are needed.

Blum also said the Pentagon will no longer break up the brigades and send them to war in smaller units. He said the Guard wants to keep brigades together because they are more effective working as a team.

All of the National Guard's combat units have been deployed to Iraq or Afghanistan as a full brigade except South Carolina's 218th Infantry. Smaller groups of its soldiers have been mobilized periodically for homeland defense and numerous missions abroad, including Iraq.

Blum said the remainder of the 218th is preparing to go to Afghanistan next year, if needed.