Obama prepared to leave no troops in Afghanistan after 2014 --- (Reuters) - U.S. President Barack Obama has told the Pentagon to prepare for the possibility that the United States will not leave behind any troops in Afghanistan after its troop drawdown at the end of this year, the White House said on Tuesday. -- Obama said he had given the order to the Pentagon in a phone call on Tuesday to Afghan President Hamid Karzai, who has refused to sign a bilateral security agreement that the United States insists it must have before agreeing to leave a contingent of troops behind. -- Defense Secretary Chuck Hagel was carrying the modified U.S. position to Brussels for discussion during a meeting with NATO defense ministers that starts on Wednesday. -- "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. -- The United States has held out the possibility of leaving behind in Afghanistan as many as 8,000 troops after the formal drawdown at year's end. These troops would conduct counter-terrorism operations against al Qaeda targets and train Afghan forces. -- Karzai's refusal to sign a security deal has frustrated the White House, which has been forced to abandon an earlier demand that the Afghan president sign the deal in weeks, not months. -- Staking out a new position, the White House statement said "we will leave open the possibility of concluding a BSA (bilateral security agreement) later this year. However, the longer we go without a BSA, the more challenging it will be to plan and execute any U.S. mission." -- And the longer both countries go without a security deal, "the more likely it will be that any post-2014 U.S. mission will be smaller in scale and ambition," the White House statement said. -- Hagel said planning for what is known as "the zero option" is a prudent step given that Karzai has made clear he is unlikely 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 United States and NATO," Hagel said in a statement. - More