US Secretary of State John Kerry has begun talks in Afghanistan, which are expected to focus on a stalled security agreement between the two nations.
Mr Kerry and Afghan President Hamid Karzai are discussing the deal which would allow the presence of US troops after the 2014 Nato troop withdrawal.
But disagreements remain over key issues, including how much the US would defend Afghanistan if it is attacked.
Mr Karzai has accused Nato of failing to bring stability to Afghanistan.
"On the security front 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 told the BBC Newsnight earlier this week.
Mr Kerry began talks with his Afghan counterpart shortly after arriving on an unannounced visit to Kabul on Friday.
Washington wants the deal done before Afghanistan's presidential election campaign opens next month.
"(US) President (Barack) Obama and President Karzai reaffirmed both back in January that the goal here was to complete the BSA (Bilateral Security Agreement) in October," a US state department official travelling with Mr Kerry was quoted as saying by the AFP news agency.
"Uncertainty about an incomplete BSA could erode the resolve among Nato allies, makes (it) more difficult to plan for the US," the official said.
But there is deadlock over how much the US would defend Afghanistan if it is attacked, with Washington not wanting to be committed to defending the mountain passes of the east against infiltration from Pakistan.
Meanwhile, Kabul wants an end to raids on its soil and is demanding that the US hand over intelligence instead.
During his visit Mr Kerry will hope to bridge what looks like a wide gap between the two sides, the BBC's Afghanistan correspondent David Loyn reports.
But he adds that US officials are now talking about the possibility of a "zero option" - no troops in Afghanistan after the end of 2014.
Mr Karzai told the BBC's Newsnight this week that he was in no hurry to sign a pact.
"If the agreement doesn't suit us then of course they can leave. The agreement has to suit Afghanistan's interests and purposes. If it doesn't suit us and if it doesn't suit them then naturally we will go separate ways."
He also claimed that Nato had incorrectly focused the fight on Afghan villages rather than Taliban safe havens in Pakistan.
Mr Karzai has had troubled relations with his Western backers in recent years for openly criticising Nato, whom he has accused of having no respect for Afghan sovereignty.
He is concerned about his legacy, not wanting to be remembered for losing Afghan sovereignty, our correspondent adds.