US Defense Secretary Chuck Hagel flew into Kabul on Saturday amid frustration in Washington over the Afghan president's refusal to sign an accord allowing NATO troops to stay in the country beyond next year.
US officials said no meeting was scheduled with President Hamid Karzai, but the president's spokesman said Karzai and Hagel were due to hold talks later Saturday -- the latest sign of discord between the allies.
Washington and its allies have appealed to Karzai to sign the Bilateral Security Agreement (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.
"Secretary Hagel does not plan to meet with President Karzai while in Afghanistan," a senior US defence official told reporters at the start of Hagel's unannounced visit.
"The United States has made its position on the BSA clear. And just two days ago, President Karzai repeated his position to senior US officials that he is not yet ready to sign the BSA and provided no timeline or practical step for doing so."
But Karzai's spokesman Aimal Faizi told AFP that the Afghan president was expecting to hold talks with Hagel -- setting the stage for another diplomatic row after a long series of public disagreements.
"In our schedule for the president today we have a meeting with Secretary Hagel this evening with the president followed by a dinner with him," Faizi said, adding that Afghanistan was keen to discuss sticking points over the BSA.
"If there is any story around that the guest will be in Kabul but will not meet with the president that is false," Faizi said.
With the Afghan election due in April, US and NATO military officers say they are running short on time to plan for a post-2014 force.
Pentagon spokesman Carl Woog confirmed Hagel would meet with Defence Minister Bismillah Khan Mohammadi and Interior Minister Omer Daudzai during his visit.
"Secretary Hagel is travelling to Afghanistan this weekend to thank troops for their service fighting far away from home and commend the progress they have made this year in advancing the capabilities of the Afghan National Security Forces," Woog added.
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.