It would be the first time such units would be returned to the war.

The plan to remobilise those reserve forces is designed to relieve growing strain on active-duty marines.

A Marine Corps spokesman, Lieutenant-Colonel Scott Fazekas, said on Wednesday that Donald Rumsfeld, the US defence secretary, gave the marines the go-ahead to conduct detailed planning on how the battalion reactivations would be done. Initially, Fazekas said Rumsfeld had approved the plan itself. Later he said the approval was for detailed planning.

Eric Ruff, a spokesman for Rumsfeld, said on Wednesday evening that no specific proposals have been presented to Rumsfeld. "The marines are reviewing a range of options and concepts for future consideration by the secretary and, to date, nothing has been approved," Ruff said.

The army, which is organised differently from the Marine Corps, has not sent any of its National Guard combat brigades back to Iraq for a second tour, although it is considering making more use of the Guard. Both the marines and army have sent reserve support units and active-duty forces to Iraq multiple times.

Return date

The return of Marine Reserve combat battalions to Iraq would begin in 2008, according to a senior Marine Corps officer who discussed the subject on condition he not be identified because no official announcement has been made.

Thus, the first picked to go back probably would be remobilised next year to train for the mission.

The plan, put forward by General Michael Hagee, the Marine Corps commandant, could be modified as the situation in Iraq changes, officials said. For planning purposes, the marines are working out future force rotations that would include at least one reserve combat battalion starting in 2008.

The marines have decided to take this unusual step in order to alleviate a problem that both the marines and the army are wrestling with as the Iraq war rages unabated: wear-and-tear on the active-duty troops, who are getting far less time at home to recuperate and retrain than military leaders would like.

Morale issue

The short respites between combat tours are not only a morale issue but also an obstacle to providing soldiers and marines with sufficiently varied training and adequate time to attend professional development schools.

The marines, for example, are not doing as much training for large-scale, high-intensity combat - combining their air, land and sea forces - as they normally would do, officials said. They do, however, have the time to do high-intensity combat training on a smaller scale and counter-insurgency training for Iraq missions.

The Marines Corps has 24 active-duty combat battalions. At any given time, nine of them are in Iraq.