WASHINGTON — Up to three more combat brigades could be available to go to Afghanistan beginning next spring, in answer to repeated calls from commanders for more troops, Defense Secretary Robert Gates said Tuesday.

Gates told the Senate Armed Services Committee that more forces can't be committed now without extending combat tours or changing troop deployments. But, in response to prodding from the committee's chairman, Sen. Carl Levin, D-Mich., Gates said they probably could go in the spring and summer of 2009.

Levin objected to a statement in Gates' prepared testimony that said it now may be "possible" to do militarily what must be done in Afghanistan — which has been a secondary priority to the Iraq war for years.

"It seems to me that is just simply not good enough," said Levin. "To say it's possible that we'll do what we must do in Afghanistan does not represent the kind of commitment of forces or resources that our commanders on the ground are asking us for in Afghanistan."

In response, Gates offered the likely troop buildup next spring, but cautioned that the next president will have to weigh how large a U.S. force should be sent to Afghanistan, considering that the population does not readily welcome foreign forces there.

"I think we need to think about how heavy a military footprint the United States ought to have in Afghanistan," said Gates, or "are we better off channeling resources into building and expanding the size of the Afghan national army as quickly as possible."

The military shortfall in Afghanistan has been a common complaint from commanders. While the number of U.S. troops in Afghanistan has grown from fewer than 21,000 two years ago to more than 31,000 today, the senior U.S. general there said last week that he needs at least 10,000 more ground troops, beyond the 3,700 Army soldiers due early next year.

The requirements include more helicopters, combat troops, trainers and other support forces. But with about 151,000 forces committed in Iraq, the U.S. has not had the available troops to send to Afghanistan. Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, has often noted that in Afghanistan "we do what we can, in Iraq we do what we must."