Three more combat brigades could be available to go to Afghanistan by the spring of 2009, the U.S. Senate armed services committee heard on Tuesday.

U.S. Defence Secretary Robert Gates told the committee more forces can't be committed now because it would require extending combat tours and changing troop deployments. But troops could probably be ready to go by spring or summer 2009, Gates said in response to questioning from committee chairman Democrat Carl Levin.

"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."

The United States has about 33,000 troops in Afghanistan.

Recently there have been repeated calls from U.S commanders for more troops.

Last week, the senior U.S. general in Afghanistan said he needs at least 10,000 more ground troops, in addition to the 3,700 army soldiers due early next year.

Gates said the next president will have to decide how large a U.S. force should be sent to Afghanistan.

"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 channelling resources into building and expanding the size of the Afghan National Army as quickly as possible."

During his prepared testimony, Gates painted a sombre picture of the hurdles in Afghanistan as the U.S. heads toward its eighth year at war in the country.

Gates said the U.S. must encourage Afghanistan and Pakistan to work together to secure their border, which has seen an increase in deadly clashes.

The violence in the region has been exacerbated by tensions over cross-border incursions by U.S. troops, as well as reports of civilian casualties, and unconfirmed suggestions that U.S. helicopters have been targeted by Pakistanis during border operations.

With files from the Associated Press