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, given 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."
Gates sketched out a complex challenge in Afghanistan in the coming months, where he said the U.S. must listen more carefully to Afghan leaders and work harder to avoid civilian casualties, which inflame the population against the military forces they may see as occupiers.
It will be important to bolster local and provincial governments without creating warlords or other militias in the process.
In other remarks, Gates signaled a turning point in Iraq, saying the U.S. has now entered the endgame there. But he said the progress should not prompt U.S. leaders to abandon caution.
Both he and Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff, told the panel that military commanders believe they need to move cautiously as they cut troop levels in Iraq.
"We do not want to jeopardize the gains that we made. We paid a high price for them," said Cartwright.
He added that support forces are needed in both Iraq and Afghanistan. And building up the U.S. force in Afghanistan depends to some degree on how quickly those support troops can be freed up.
Members of the panel pressed Gates on U.S. military operations along the Pakistan and Afghanistan border, and the often tense relations among the three countries.
Gates said Afghan leaders he met with last week spoke more optimistically about relations with Pakistan.
He also noted that U.S. relations with Pakistan are critical since about 80 percent of the military's cargo supplies and 40 percent of the fuel are transported through Pakistan and across the border.