US President Barack Obama shakes hands with troops after delivering remarks at Bagram Air Base in Kabul, May 25, 2014. [Photo/Agencies]

At Bagram, Obama addressed the troops just after a performance from country music star Brad Paisley, who joined him for the Air Force One ride halfway around the world, a trip kept under wraps until the plane arrived.

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"Once Afghanistan has sworn in its new president, I'm hopeful we'll sign a bilateral security agreement that lets us move forward," Obama told the troops.

"We want to preserve the gains that you have helped to win. And we're going to make sure that Afghanistan can never again, ever, be used again to launch an attack against our country."

Obama's remarks were laced with patriotic praise for troops ahead of the US Memorial Day on Monday, a holiday of remembrance for those Americans who lost their lives in wars.

"I know sometimes when you're over here, away from home, away from family, you may not truly absorb how much the folks back home are thinking about you. So I just want you to know, when it comes to supporting you and your families, the American people stand united," he said.

Criticized at home for his handling of a controversy involving healthcare delays for veterans, Obama vowed his administration will uphold the "sacred obligation" to take care of those who had served in the US military.

Obama got a briefing from the Army general who commands US and NATO forces here, Joe Dunford, and US Ambassador to Afghanistan James Cunningham. He said he would announce "fairly soon" how many troops the United States would like to leave behind.