The top Republican on the Senate committee, John McCain voiced doubt about Obama's withdrawal plan in Afghanistan, echoing fears that it could allow Taliban militants to wait out the U.S. troop surge and reassert themselves later.

McCain said the goal of securing Afghanistan and eliminating safe havens for al Qaeda extremists was admirable, but an arbitrary U.S. pullout date was dangerous.

A date for withdrawal sends exactly the wrong message to both our friends and our enemies, he said.

President Barack Obama announced Tuesday the U.S. will send 30,000 more troops into Afghanistan to fight against the Taliban insurgents in order to allow U.S. soldiers to start pulling out by the summer of 2011.

Many of Obama's fellow Democrats have voiced doubt about escalating the costly conflict, while Republicans have complained that the drawdown date ties the military's hands.

It will be very challenging. There will be nothing easy about it. There has been nothing easy. Afghanistan is hard and it's hard all the time and we have our eyes wide open about that, said U.S. Army General David Petraeus, who now commands U.S. forces in the whole region.

Gates and other top officials, under sharp questioning by Republicans, suggested that the 18-month withdrawal timeline could change if circumstances on the ground indicate the fight is not being won.

I do not believe we have locked ourselves into leaving, Secretary of State Hillary Clinton told the committee, saying the United States wanted to show it did not intend to occupy Afghanistan indefinitely.

We're not interested in running their country, building their nation. We are trying to give them the space and time to be able to build up sufficient forces to defend themselves, she said.

Clinton said Washington would press Afghan President Hamid Karzai to deliver on promises to fight corruption and would support moves to bring in moderate elements of the Taliban which renounce violence.

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