President Barack Obama has warned his Afghan counterpart Hamid Karzai that the US may pull all of its troops out of his country by the year’s end.
Mr Obama conveyed the message in a phone call to Mr Karzai, who has refused to sign a security agreement.
The US insists this agreement must be in place before it commits to leaving some troops behind for counter-insurgent operations and training.
The US has had troops in Afghanistan since 2001 when it toppled the Taliban.
Its forces went into the country following the 9/11 attacks on the US. With Afghan and Western allies, they quickly overthrew the Taliban authorities, but have faced insurgent attacks since then.
Correspondents say the disagreement over the bilateral security agreement (BSA) is the latest step in the long and deteriorating relationship between Washington and Mr Karzai, who was once seen as a key US ally.
The BSA, which offers legal protection for US troops and defines a post-2014 Nato training and anti-insurgent mission, was agreed by the two countries last year after months of negotiation.
It was endorsed at a national gathering (Loya Jirga) of Afghan elders in Kabul in November.