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.

NATO must plan for all options, including a complete withdrawal from Afghanistan, but prefers an agreement with Kabul on a continued troop presence, NATO chief Anders Fogh Rasmussen said Wednesday.

"If there is no US-Afghan agreement, there can be no NATO-Afghan deal ... and if there is no agreement, there will be no NATO troops in Afghanistan after 2014," Rasmussen said.

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.

"President Obama told President Karzai that because he has demonstrated that it is unlikely that he will sign the BSA, the United States is moving forward with additional contingency planning," a White House statement said, detailing the call.

"Specifically, 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."

The White House had previously warned Karzai's intransigence on a deal 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.

White House spokesman Jay Carney however said Washington was not certain a future government would be on board.

In Kabul, presidential spokesman Aimal Faizi told AFP that the conversation lasted 40 minutes and was friendly.

He said Karzai told Obama that Afghans wanted the BSA signed - but restated his condition that Washington must first bring the Taliban into peace talks.

AFP