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President Barack Obama slipped into Afghanistan unannounced Sunday in a six-hour visit to rally thousands of U.S. troops and press his Afghan counterpart to move more quickly to fight government corruption.

The trip, Mr. Obama's first to the war zone since taking office 14 months ago, was shrouded in secrecy for security reasons. The White House informed Afghan President Hamid Karzai of the visit on Thursday. Mr. Obama was ostensibly spending the weekend with his family at Camp David, Md., when he took off for Afghanistan on Air Force One on Saturday night.

Administration officials said Mr. Obama took a hard line with Mr. Karzai -- whose government has been on shaky ground since winning a disputed election -- over corruption and drug trafficking.

The two leaders also discussed ongoing efforts to make peace with willing insurgents and reintegrate them into Afghan society. After a brief meeting at the Presidential Palace in which Mr. Karzai thanked U.S. taxpayers for rebuilding his country, Mr. Obama moved on to a visit with U.S. troops.

The president, wearing a bomber jacket, spoke for about 20 minutes to thousands of cheering troops inside a cavernous tent at the U.S. base in Bagram.

'My main job here today is to say thank you on behalf of the entire American people,' he told the soldiers, hundreds of whom held cameras above their heads. 'You are bringing hope and opportunity to a people who know a lot of pain and suffering.'

He said progress has been made in the past few months of the troop surge and again stated the case for committing U.S. troops here, saying America was 'viciously attacked' by people who 'operate within the confines of Afghanistan.'

Mr. Obama then met with wounded servicemen in a nearby medical clinic and stopped by a chow hall to chat with about 50 others.

The visit comes just weeks after the first major offensive of the surge of U.S. reinforcements in Afghanistan: a drive into the southern town of Marjah, which had become a Taliban stronghold. Thus far, the Marjah operation has been seen as a success, with U.S. Marines retaking the town with fewer casualties than anticipated.

The biggest test of the military strategy is yet to come, however. The main fight of the surge is set for this summer in the Taliban's spiritual home of Kandahar, in southern Afghanisatan.