WASHINGTON - Defence Secretary Robert Gates told a Senate panel Tuesday that the Pentagon could send two more brigades to Afghanistan by late spring and a third brigade by late summer in an effort to try to salvage a country besieged by corruption and increasing violence.

More troops could be sent once the Defence Department is able to put a larger infrastructure in place to support them, he added in testimony to the Senate Armed Services Committee.

Gates' prediction comes as President Barack Obama considers his options for a drawdown of troops in Iraq.

The Pentagon is preparing various scenarios for winding down the war, including a plan that would cease U.S. involvement in combat within 16 months. Gates said military planners are looking at later dates as well and are prepared to brief Obama on all his options and the their associated risks.

Obama planned to meet Wednesday with the service chiefs, who are helping to prepare various scenarios for winding down the war.

"I believe the president will have had every opportunity to hear quite directly from his commanders about what they can accomplish and what the attendant risks are under different options," Gates said.

It is Gates' first hearing since Obama took office and legislators were eager to hear details about how the new president planned to turn around the war in Afghanistan.

"This is a long, hard slog we're in Afghanistan," said Sen. John McCain of Arizona, borrowing the phrase used frequently by former defence secretary Donald Rumsfeld to describe the war in Iraq.

"It is complex," added McCain, the top Republican on the Armed Services Committee. "It is challenging. And I don't see frankly an Anbar wakening -- a game changing event -- in Afghanistan, such as we were able to see in Iraq."

Security gains made in Iraq's Anbar province are often seen as a turning point in the Iraq war.

Afghanistan is America's "greatest military challenge" and co-ordination of the fight against the insurgency has been "less than stellar," Gates said in his prepared remarks.

He said it will take a long and difficult fight to rout militants and help develop a country that rejects the Islamic fundamentalist Taliban and backs its own elected government.

"There is little doubt that our greatest military challenge right now is Afghanistan," Gates said.