President Hamid Karzai and US Secretary of State John Kerry said Saturday that talks on the future of US forces in Afghanistan were stuck on the key issue of US troop immunity.
Kerry extended his stay in Kabul to try to thrash out a long-delayed security pact that would allow between 5,000 and 10,000 US troops to remain in Afghanistan after 2014 to fight Al-Qaeda remnants and train the national army.
But he said that a major sticking point in efforts to sign the Bilateral Security Agreement (BSA) was the issue of which country would try any US soldiers deployed in Afghanistan.
"One issue that is outstanding (is) the issue of jurisdiction," he said, rejecting the widely-used term "immunity" because accused US troops would still stand trial in America.
"We need to say that if the issue cannot be resolved, unfortunately there cannot be a bilateral security agreement."
Karzai said that a national assembly of tribal elders would be called to discuss whether foreign soldiers could be given immunity from prosecution in Afghanistan, as the issue was "above government authority".
"After several months of negotiation, and intense talks yesterday and today, we have reached a series of agreements," Karzai told reporters at a joint press conference in Kabul.
"The BSA has a lot of items, one is about immunity for foreign and US soldiers -- we didn't have a united opinion on this issue."
A similar US security agreement with Iraq in 2011 collapsed over the issue of troop immunity.
The US pulled its troops out of the country, which is currently suffering its worst sectarian violence since 2008.
But Afghan officials dismiss the possibility that the US may enact the "zero option" of a complete pull-out after its soldiers have fought the Taliban militants since 2001.
Kerry had been due to fly to Paris on Saturday morning, but negotiations ran late into the evening.
A US official said Kerry wanted "to leave Kabul with as many issues resolved as possible".
The US wants the security deal signed within weeks to enable the NATO military coalition to plan its withdrawal of 87,000 combat troops from Afghanistan by December 2014, but Karzai recently threatened to walk away from talks.
Karzai said on Saturday that progress was made on other major points of dispute, including the US agreeing not to conduct unilateral military operations against militants after 2014.
"There will be no arbitrary actions and operations by the US, and a written document has been given to guarantee the protection of lives and properties of our people," the president said.
Karzai 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 international military efforts to thwart Taliban insurgents.
He 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 to choose Karzai's successor is seen as the key test of Afghan stability as NATO troops withdraw, and Kerry stressed the US would support a free and fair vote.
Karzai 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 the US and Afghan governments now back peace talks to end the conflict.
Kerry left Kabul on Saturday evening and was due to travel to London after Paris.