WASHINGTON - US President Barack Obama, in a phone conversation with Afghan President Hamid Karzai on Tuesday, raised the possibility of no American troops left in Afghanistan beyond 2014.

Karzai has refused to sign a bilateral security agreement (BSA) that allows for and grants legal immunity to residual American troops, who will be tasked with training and assisting Afghan forces as well as conducting counterterrorism operations beyond 2014 after most American and NATO combat troops exit.

Obama told the Afghan leader that Washington was moving forward with "additional contingency planning".

"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 said in a readout of the phone talks.

Obama left open the possibility of concluding the security pact with Afghanistan later this year, as a new Afghan president will emerge in elections slated for April.

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

Row over the pact is just one irritant in Washington-Kabul ties, as Karzai had set new conditions for his signing of the deal, including a promise by Washington not to allow its forces to conduct counterterrorism raids on Afghan homes, and US agreement to free Afghan prisoners held at its Guantanamo prison.

US Secretary of Defense Chuck Hagel said Tuesday that his department will move ahead with "additional contingency planning" to ensure adequate plans are in place for an orderly withdrawal by the end of the year.

In a statement, the Pentagon chief called the potential "zero option," in which no American troops will be left behind in Afghanistan after 2014, "a prudent step" in face of Karzai's unwillingness to sign the security deal.

"As the United States military continues to move people and equipment out of the Afghan theater, our force posture over the next several months will provide various options for political leaders in the Untied States and NATO," Hagel said.

He will join other NATO defense ministers for meetings in Brussels on Wednesday and Thursday, with Afghanistan atop the agenda.