WASHINGTON: President Barack Obama warned his Afghan counterpart Hamid Karzai on Tuesday that the US might pull all of its troops out of Afghanistan by the end of this year and ordered the Pentagon to prepare for a complete withdrawal, the White House said.

It blamed President Karzai's continued refusal to sign a bilateral security agreement (BSA) with the United States for the situation but left open the possibility of concluding the agreement with a new Afghan president later this year.

The Pentagon also issued a statement saying that US Defence Secretary Chuck Hagel had ordered his troops to plan for “an orderly withdrawal” by the end of the year should the US government decide to pull out of Afghanistan.

The White House said that President Obama called President Karzai in the day to discuss preparations for Afghanistan's coming elections, Afghan-led peace and reconciliation efforts and the bilateral security agreement.

The two leaders discussed the possibility of Afghanistan signing a security agreement with the United States before the Nato defence ministerial meeting, which begins on Wednesday. Secretary Hagel flew to Brussels on Tuesday to attend the meeting, which will discuss Nato's future plans for Afghanistan.

“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 White House 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.”

But the White House also indicated that it would be willing to sign the agreement with a new Afghan leader to be elected in April after President Karzai completed his second and final term next month.

“At the same time, should we have a BSA and a willing and committed partner in the Afghan government, a limited post-2014 mission focused on training, advising, and assisting Afghan forces and going after the remnants of core Al Qaeda could be in the interests of the United States and Afghanistan,” the White House said.

“Therefore, we will leave open the possibility of concluding a BSA with Afghanistan later this year.”

The White House warned President Karzai that the longer the delay in signing a BSA, the more challenging it would 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.”

Secretary Hagel's statement, however, focused on the withdrawal, saying that “at President Obama's direction, and with my strong support, the Department of Defence will move ahead with additional contingency planning to ensure adequate plans are 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”.

Mr Hagel called it “a prudent step” given President Karzai's continued refusal to sign the agreement that will provide critical protections and authorities to the troops that stay in Afghanistan after 2014.