In addition, Defense Secretary Robert Gates has authorized the deployment of up to 200 other troops to Afghanistan to support the Marines. That includes eight helicopter crews that could be shifted from Iraq if commanders decide.
The senior military official spoke to The Associated Press on Monday on condition of anonymity because the formal announcement has not yet been made.
The decision to extend the tour of the 2nd Battalion, 7th Marine Regiment in Afghanistan comes just a month after defense officials told the 24th Marine Expeditionary Unit that it wouldin Afghanistan.
According to the official, the decision to hold the battalion there longer is part of an effort to capitalize on the gains the Marines have made in the training mission. The extension means that the battalion would return home in late November.
Asked about Gates' decision, Pentagon press secretary Geoff Morrell said the secretary was responding to a request from the commanders.
Gates "is always pained to have to extend tours. He understands the effect that has on the families of our forces, but he also appreciates our commanders' need to make additional progress while the weather is still good in Afghanistan," Morrell said.
Gates' decision to send the other support forces comes after weeks of discussions by top military leaders who scrambled to find needed troops. He authorized Army Lt. Gen. Martin E. Dempsey, who is temporarily in charge of U.S. Central Command, to shift up to eight helicopter units from Iraq to Afghanistan - four Cobra attack aircraft and four MH-53 heavy lift helicopters.
The remainder of the support forces being deployed are smaller units, including engineers, route clearance troops and explosive ordnance disposal teams. It was not clear Monday whether those support forces also would return home in late November, or if they would stay longer in Afghanistan.
The Pentagon announced in January that the Marine Expeditionary Unit, which is based at Camp Lejeune, N.C., was being ordered to Afghanistan, largely because efforts to press other NATO nations to increase their troop levels at the time had failed. The MEU has been fighting Taliban militants in the volatile south.
At the same time, the 2nd Battalion, 7th Marine Regiment, which is based at Twentynine Palms, Calif., was ordered to deploy also.
Gates has said he would not replace the Marines with other U.S. troops when they left later this year. But commanders have said they need three more combat brigades - or as many as 10,000 troops - to bolster the fight in Afghanistan. And U.S. officials have indicated they would like to send extra brigades there next year.
Morrell told a news conference on July 23 that the decision on how and when to meet the request for the larger amount of troops was "a question, frankly, for the next administration," which will be chosen in November's presidential election.
Military leaders, however, have made it clear they need to free units from Iraq deployments in order to send more troops to Afghanistan. As security in Iraq continues to improve, officials have suggested that units initially headed for Iraq late this year or early next year could be sent to Afghanistan instead.