President Barack Obama told his Afghan counterpart Hamid Karzai on Tuesday he is now planning a full US troop withdrawal, but did not rule out agreeing a post-2014 mission with the next Kabul government.

The US threat was the latest twist in a long political tug-of-war with Karzai, who has infuriated Washington with his refusal to sign a Bilateral Security Agreement (BSA) governing a post-2014 mission in the dying months of his mandate.

"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," the statement said.

"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 has previously warned that Karzai's intransigence on a deal painstakingly negotiated last year meant it had no choice but to consider a full withdrawal.

It refuses to leave troops behind in Afghanistan after America's longest war without the legal protections granted by the BSA.

Though Karzai has refused to sign the pact, which defines a post-2014 NATO training and anti-terror mission, some of the candidates in April's Afghan elections have indicated they would sign it. The deal has also been endorsed by a council of tribal elders.

The statement said that Obama was reserving the "possibility of concluding a BSA with Afghanistan later this year," should he find a willing partner in the government.

It was the most concrete sign yet that Washington may be willing to wait out the Afghan electoral process before making a final decision on a future role in Afghanistan.

However, the statement warned "the longer we go without a BSA, the more challenging it will be to plan and execute any US mission."

"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."

The statement clearly implied that Karzai's behavior would harm his nation's security long into the future.

Defense Secretary Chuck Hagel backed Obama's move, and confirmed for the first time the Pentagon was actively planning a full troop withdrawal.

Hagel said that top Pentagon brass would simultaneously plan options for a full withdrawal and a prolonged mission in Afghanistan, which would likely to include at least several thousand US troops.

The row over the BSA is the latest twist in the long and deteriorating relationship between Washington and the mercurial Karzai, who was once seen as a savior after the toppling of the Taliban but is now viewed in political and security circles here as unreliable and unpredictable.

Obama has not yet made a decision on how many troops would be left behind to train and equip Afghan forces after the withdrawal of US combat teams by the end of this year, should he decide on a follow up mission.

Obama's political opponents have warned that leaving Afghanistan without Western troops would severely strain the fledgling national forces stood up by NATO and could lead to a return by the Taliban.

Some have compared such a scenario to Washington's loss of focus after helping rebels oust Soviet occupiers in the 1980s, leaving a power vacuum exploited by the Taliban, which eventually offered haven to Al-Qaeda as it planned the September 11 attacks in 2001.

The White House said Obama used Tuesday's call with Karzai, who is not standing in the election, to welcome the start of campaign season and to push for free, fair and credible polls.

"The President reiterated that the United States would not support any candidate in the elections -- the choice of who leads Afghanistan is for Afghans to make," the statement said.