American president Barack Obama has asked the Pentagon to make plans to withdraw all United States troops from Afghanistan by the end of this year, if a security agreement is not signed.
Mr Obama conveyed the message in a phone call to Afghan president Hamid Karzai, who has so far refused to sign.
The United States insists the agreement must be in place before it commits to leaving some troops behind for counter-insurgent operations and training.
US forces went into Afghanistan in 2001, after the 9/11 attacks on the US, and with Afghan and Western allies they quickly overthrew the Taliban authorities. But insurgent attacks have continued ever since.
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.
But Mr Karzai has refused to sign it until a peace process is under way with the Taliban, adding that if he were to sign it, he would become responsible if Afghans were killed by US bombs.
The White House says Mr Obama told Mr Karzai that because he has demonstrated that it is unlikely that he will sign the BSA the US is "moving forward with additional contingency planning".
"Specifically," it goes on, "President Obama has asked the Pentagon to ensure that it has adequate plans in place to accomplish an orderly withdrawal by the end of the year should the United States not keep any troops in Afghanistan after 2014.
"Furthermore, the longer we go without a BSA, the more likely it will be that any post-2014 US mission will be smaller in scale and ambition."