President Barack Obama told Hamid Karzai on Tuesday that he is now planning for a full US troop withdrawal because of the Afghan leader's repeated refusal to sign a security pact.
But in a rare telephone call with President Karzai, Obama also held out the possibility of agreeing a post-2014 training and anti-terror mission with the next government in Kabul.
The US threat was the latest twist in a long political struggle with Karzai, who appears intent on infuriating Washington until the day he leaves office, sometime after elections in April.
The Obama administration said its preferred option is to leave behind a residual US force when its combat teams depart Afghanistan after America's longest war at the end of this year.
But it will not do so without legal protections enshrined in the Bilateral Security Agreement (BSA) agreed between the two governments, which Karzai will not endorse.
The White House had previously warned that Karzai's intransigence on a deal painstakingly negotiated last year meant it had no choice but to mull the "zero option."
The statement said Obama was reserving the possibility of concluding a BSA with Afghanistan later this year should the new government be willing.
It was the most concrete sign yet that Washington could wait out the Afghan electoral process before making a final decision on a future role in Afghanistan.