The U.S. says it might leave no troops in Afghanistan after December 2014, an option that defies the Pentagon's view that thousands of troops may be needed to contain al-Qaeda and to strengthen Afghan forces.

"We wouldn't rule out any option," including zero troops, Ben Rhodes, a White House deputy national security adviser, said Tuesday.

U.S. President Barack Obama, left, and Afghan President Hamid Karzai walk together after signing a strategic partnership agreement in Kabul, Afghanistan. The two leaders are scheduled to meet Thursday to discuss their strategic partnership beyond 2014. (Charles Dharapak/Associated Press)

"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 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 U.S. now has 66,000 troops in Afghanistan, down from a peak of about 100,000 as recently as 2010. The U.S. 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.

Those issues are at the top of the agenda in talks this week as Afghan President Hamid Karzai meets with U.S. President Barack Obama on Friday, and Defence Secretary Leon Panetta and Secretary of State Hillary Rodham Clinton on Thursday.

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.

Troops presence could go down to 3,000

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

Fred Kagan, a military historian and Afghan war specialist at the American Enterprise Institute, wrote in a Wall Street Journal opinion piece Wednesday that reducing the U.S. military presence as much as the White House is suggesting would, in effect, waste what has been achieved in a decade of fighting in Afghanistan.

"The Obama administration will no doubt promise that the U.S. will continue to provide assistance to the Afghan military in addition to continuing counterterrorism operations in Afghanistan. But the military reality is that we cannot conduct either mission at the force level the president is considering," Kagan wrote in the piece co-written with his wife, Kimberly Kagan, of the Washington-based Institute for the Study of War.

Administration officials in recent days have said they are considering a range of options for a residual U.S. 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 U.S. 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 U.S. support to maintain an effective security force after 2014.

Karzai to meet with Clinton, Panetta this week

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 U.S. troops.

Without explicitly mentioning immunity for U.S. troops, Obama's top White House military adviser on Afghanistan, Doug Lute, told reporters Tuesday that the Afghans will have to give the U.S. certain "authorities" if it wants U.S. 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 U.S. military mission," Lute said. He was referring to 2011 negotiations with Iraq that ended with no agreement to grant legal immunity to U.S. troops who would have stayed to help train Iraqi forces. As a result, no U.S. troops remain in Iraq.

'Intelligence war' beyond 2014

David Barno, a retired Army three-star general and former commander of U.S. forces in Afghanistan, 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 U.S. troops, Barno cautioned that Afghanistan could "slip back into chaos."

Barno said the Afghan-Pakistan border area where numbers of Islamic extremists are in hiding could become the scene of a prolonged "intelligence war" after 2014, with the U.S. and its Afghan and Pakistan partners sharing intelligence.

"Given its vital importance, this undertaking will endure — regardless of the size of the residual U.S. military presence," he wrote.

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 U.S. troops — if any — he wants to keep there.

Rhodes said Obama remains committed to further reducing the U.S. military presence this year, although the pace of that withdrawal will not be decided for a few months.