Having no troops in Afghanistan -- known as the "zero option" -- "would be an option that we would consider" when the U.N. security mandate sanctioning the international coalition expires next year, deputy national security adviser for strategic communication Ben Rhodes told reporters in a conference call.
"The U.S. does not have an inherent objective of X number of troops in Afghanistan," Rhodes said. "We have an objective of making sure there's no safe haven for al-Qaida within Afghanistan and making sure that the Afghan government has a security force that is sufficient to ensure the stability of the Afghan government and the denial of that safe haven.
"So that's what guides us and that's what causes us to look for different potential troop numbers, or not having potential troops in the country," he said.
The administration might find non-military ways of meeting U.S. objectives in Afghanistan, he said.
The Washington Post said some groups within the administration are pushing to shrink U.S. forces to no more than a few thousand troops -- perhaps as few as 2,500 -- after 2014, arguing a small number would be the most constructive way of capping the costly, unpopular war.
The low number is far less than the 10,000 to 30,000 troops discussed among NATO allies and some U.S. officials as recently as a year ago, the Post said.
Some senior military officials have pressed for a more robust figure, arguing a hurried withdrawal would be irresponsible and could cause Afghanistan's security forces to collapse.
A recent Pentagon report indicated only one of the Afghan National Army's 23 brigades is able to operate without support from the United States and other NATO nations.
Some 68,000 U.S. troops are currently stationed in Afghanistan.
The size and scope of a post-2014 U.S. force are widely expected to be a top Karzai agenda item during his Washington visit, which includes a meeting with President Obama and Secretary of State Hillary Clinton at the White House Friday.
Karzai is to meet with Defense Secretary Leon Panetta at the Pentagon Thursday. He is also scheduled to speak at Georgetown University about Afghanistan's future.
The White House meeting will be the first face-to-face talks between Obama and Karzai since the May 2012 NATO summit in Chicago. That summit took place some three weeks after Obama made a surprise May 1 trip to Kabul to sign a landmark strategic partnership agreement, intended to be a road map for a new relationship after the departure of U.S. troops.
The agreement pledges U.S. support for Afghanistan for 10 years after the withdrawal of the last U.S. soldiers.