US Secretary of State John Kerry held talks with President Hamid Karzai in Kabul on Friday to try to advance troubled negotiations with Afghanistan on some US troops staying in the country after 2014.
Karzai said this week that he was prepared to walk away from the Bilateral Security Agreement (BSA) talks if Afghanistan was not happy with its conditions.
The United States has repeatedly pressed for the pact to be signed by the end of this month, so that the US-led NATO military coalition can schedule its withdrawal of 87,000 combat troops by December 2014.
"President Obama and President Karzai reaffirmed both back in January that the goal here was to complete the BSA in October," a state department official travelling with Kerry told reporters.
"We continue to believe that is both preferable and doable.
"Uncertainty about an incomplete BSA could erode the resolve among NATO allies, makes (it) more difficult to plan for the US, makes (it) more difficult to plan for our NATO allies."
Karzai has said he refuses to be rushed into signing the deal, and would first seek approval from a traditional grand assembly of tribal leaders to be convened in about month's time.
"If it doesn't suit us and if it doesn't suit them, then naturally we will go separate ways," he said in a BBC interview broadcast on Monday.
The agreement would see a few thousand US troops remain in Afghanistan to train local forces and target Al-Qaeda remnants.
According to the Afghan government, talks ground to a halt over US demands for the right to conduct unilateral military operations, and on how the US would pledge to protect Afghanistan.
The collapse of a similar agreement with Iraq in 2011 led to the US pulling its troops out of the country, which is currently suffering its worst sectarian violence since 2008.
But Kabul has dismissed the possibility that the US may enact the "zero option" of a complete pull-out after its soldiers have fought the Taliban since the 9/11 attacks in 2001.
"The BSA negotiations have been going on for 11 months. We are at a pivotal period," the US official said, adding however that the deal would not be signed during Kerry's unannounced visit, which will last until Saturday.
The Afghan leader has had a tempestuous relationship with the US and other foreign allies since he came to power in 2001, often sparking outrage with his criticism of costly international military efforts to thwart the Taliban.
"The entire NATO exercise was one that caused Afghanistan a lot of suffering, a lot of loss of life, and no gains because the country is not secure," he said this week.
Karzai is due to stand down for elections in April 2014, and many analysts say he is keen to secure a reputation as a strong nationalist leader.
A credible election is seen as the key test of Afghanistan's stability as NATO troops withdraw, and US officials said Kerry would discuss issues such as voter registration and security with the president.
Karzai, who hosted Kerry at an informal dinner in Kabul on Friday, officially suspended BSA talks in June in a furious reaction to the Taliban opening a liaison office in Qatar that was presented as an embassy for a government in waiting.
The Taliban regime was driven from power by a US-led coalition in 2001 for sheltering the Al-Qaeda leaders behind the 9/11 attacks.
Since then the Islamist rebels have fought a bloody insurgency, and both the US and Afghan governments now back peace talks to end the conflict.
Kerry, who released a statement congratulating the Organisation for the Prohibition of Chemical Weapons for winning the Nobel prize, will travel to Paris on Saturday and then London.