US Defense Secretary Chuck Hagel on Saturday said he had received assurances during a visit to Kabul that a long-delayed deal allowing US troops to stay in Afghanistan after 2014 would be signed "in a timely manner".
The Bilateral Security Agreement (BSA) has been at the centre of a public dispute between the allies, with the US increasingly frustrated by President Hamid Karzai's negotiating tactics over the deal.
After meetings in the Afghan capital, Hagel told reporters that Defence Minister Bismillah Khan Mohammadi "assured me the BSA (bilateral security agreement) would be signed in a timely manner".
Washington and its allies have repeatedly appealed to Karzai to sign the BSA, which lays out the rules for US and NATO troops to operate in the country after 2014 on a mission focused on training and countering Al-Qaeda-linked extremists.
The Afghan president, who will stand down next year after two terms in power, recently refused to sign the pact promptly despite a "loya jirga" national assembly that he convened voting for him to do so.
President Barack Obama's deputies have warned that unless Karzai relents before the end of the year, there will be no option but to prepare for a full US exit -- the so-called "zero option".
With no US boots on the ground, Afghanistan would face the risk of a Taliban resurgence and likely lose out on a billions of dollars of military and other international aid.
There are currently 46,000 American troops and 27,000 soldiers from other coalition countries in Afghanistan, and almost the entire NATO-led force is scheduled to pull out by the end of next year.
Under the proposed post-2014 mission, roughly 12,000 troops -- mostly American -- would remain in the country, under rules that would allow controversial house raids by Western forces only in special circumstances.
The US military's top officer, General Martin Dempsey, said Friday he had not yet been asked to start planning for a scenario in which all US troops would be withdrawn after 2014.
"I have not been told to plan for a zero option, but clearly, I understand that it is a possibility given the current impasse," Dempsey told reporters.
Although Dempsey said the military would not face logistical constraints in planning for post-2014 options until "early summer," he said the psychological effect of further delays would undermine the confidence of Afghan forces as well as coalition countries trying to make troop plans.
Washington had initially set an October deadline for clinching the security agreement and later insisted on a signature by the end of this year.
Hagel arrived in the Afghan capital after a visit to Bahrain.