The Obama administration is debating proposals that would increase Afghan security forces by up to 73,000, as officials wonder how much Afghanistan can absorb and what the U.S. can afford.

Defense Secretary Robert Gates told the Senate Armed Services Committee on Thursday that the U.S. will spend $12.8 billion in 2012 to train and equip the Afghan forces.

"So the question is, how long can we afford to do that? And you cannot do that indefinitely," Gates said.

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said the exact range under consideration is between 352,000 and 378,000, disclosing for the first time the figures under discussion.

U.S. commanders have said it is critical to increase the size of the Afghan forces beyond the goal of 305,000 by the end of this year.

The comments from Gates and Mullen suggested there may be some disagreement within the administration and the commanders in Afghanistan over the size of the Afghan force. An important factor, they said, is the cost.

"They cannot afford a force the size that they already have," said Gates. "And so I think the only way we can think of it, or the way we ought to think of it, is something that we would be willing to support for a few years. "

Perhaps, Gates said, the Afghan force can increase temporarily until the country is more stable, and then fall back to a lower level.

He said the U.S. probably would need to keep helping the Afghans, even down the road, but "it would be a dramatically smaller bill than it is now, and if it's a smaller bill we may be able to get other countries to help us as well. "

The committee chairman, Sen. Carl Levin, D-Mich., said he strongly supports the increase to 378,000, and expressed doubts it would go much lower.

"They are going to need a significant military and a security force," said Levin. "That is the ticket to success, as well as to an exit . or at least a significantly reduced number of foreign troops."

Gates and Mullen said a decision is expected soon, and that the 2012 budget proposal would cover the growth.