"The US does not have an inherent objective of 'X' number of troops in Afghanistan,'' said Ben Rhodes, a White House deputy national security adviser. "We have an objective of making sure there is no safe haven for al-Qaeda in Afghanistan and making sure that the Afghan government has a security force that is sufficient to ensure the stability of the Afghan government.''

The US now has 66,000 troops in Afghanistan, down from a peak of about 100,000 as recently as 2010. The US and its NATO allies agreed in November 2010 that they would withdraw all their combat troops by the end of 2014, but they have yet to decide what future missions will be necessary and how many troops they would require.

At stake is the risk of Afghanistan's collapse and a return to the chaos of the 1990s that enabled the Taliban to seize power and provide a haven for Osama bin Laden's al-Qaeda network. Fewer than 100 al-Qaeda fighters are believed to remain in Afghanistan, although a larger number are just across the border in Pakistani sanctuaries.

Defense Secretary Leon Panetta has said he foresees a need for a US counterterrorism force in Afghanistan beyond 2014, plus a contingent to train Afghan forces. He is believed to favor an option that would keep about 9,000 troops in the country.

Administration officials in recent days have said they are considering a range of options for a residual US troop presence of as few as 3,000 and as many as 15,000, with the number linked to a specific set of military-related missions like hunting down terrorists.

Asked in a conference call with reporters whether zero was now an option, Rhodes said, "That would be an option we would consider.''

His statement could be interpreted as part of an administration negotiating strategy. On Friday Afghan President Hamid Karzai is scheduled to meet President Barack Obama at the White House to discuss ways of framing an enduring partnership beyond 2014.

The two are at odds on numerous issues, including a US demand that any American troops who would remain in Afghanistan after the combat mission ends be granted immunity from prosecution under Afghan law. Karzai has resisted, while emphasizing his need for large-scale US support to maintain an effective security force after 2014.

In announcing last month in Kabul that he had accepted Obama's invitation to visit this week, Karzai made plain his objectives.

"Give us a good army, a good air force and a capability to project Afghan interests in the region,'' Karzai said, and he would gladly reciprocate by easing the path to legal immunity for US troops.

Karzai is scheduled to meet Thursday with Panetta at the Pentagon and with Secretary of State Hillary Rodham Clinton at the State Department.

Without explicitly mentioning immunity for US troops, Obama's top White House military adviser on Afghanistan, Doug Lute, told reporters Tuesday that the Afghans will have to give the US certain "authorities'' if it wants US troops to remain.

"As we know from our Iraq experience, if there are no authorities granted by the sovereign state, then there's not room for a follow-on US military mission,'' Lute said. He was referring to 2011 negotiations with Iraq that ended with no agreement to grant legal immunity to US troops who would have stayed to help train Iraqi forces. As a result, no US troops remain in Iraq.

David Barno, a former commander of US forces in Afghanistan and now a senior fellow at the Center for a New American Security, wrote earlier this week that vigorous debate has been under way inside the administration on a "minimalist approach'' for post-2014 Afghanistan.

In an opinion piece for ForeignPolicy.com on Monday, Barno said the "zero option'' was less than optimal but "not necessarily an untenable one.'' Without what he called the stabilizing influence of US troops, Barno cautioned that Afghanistan could "slip back into chaos.''

Rhodes said Obama is focused on two main outcomes in Afghanistan: ensuring that the country does not revert to being the al-Qaeda haven it was prior to Sept. 11, 2001, and getting the government to the point where it can defend itself.

"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,'' Rhodes said.

He predicted that Obama and Karzai would come to no concrete conclusions on international military missions in Afghanistan beyond 2014, and he said it likely would be months before Obama decides how many US troops _ if any – he wants to keep there.