WASHINGTON — President Barack Obama is prepared to extend a Dec. 31 deadline in a concession to Afghan President Hamid Karzai aimed at getting him to approve a security agreement that would permit U.S forces to stay in Afghanistan past 2014, aides say.

The White House has warned for months that all U.S forces will be withdrawn unless a deal is reached, and top advisers to Obama are increasingly comfortable with that prospect. At least two senior officials say the so-called zero option is strategically viable and politically acceptable, although it still isn't the preferred outcome.

Support among Obama's senior staff has grown for a full withdrawal despite objections from U.S commanders in Afghanistan and at the Pentagon, who warn that hard-won gains by U.S forces over the last 12 years could be reversed amid the still-bitter insurgency.

Both sides point to Iraq, which has fallen into sectarian violence since the White House withdrew all U.S forces in 2011 after the government there refused to sign a similar deal. But the bloodletting in Iraq has barely been noticed by an American public relieved to leave the unpopular war behind.

Obama remains committed to the Afghan security agreement and wants the strategic planning to begin now. But several aides said he had agreed in principle to let the Dec. 31 signing deadline slide for several weeks.

Some in Obama's inner circle are so exasperated with Karzai that they are willing to wait until the Afghan presidential election is held April 5, as Karzai has demanded, hoping his successor will then sign the pact. That option remains under consideration, but Obama is unlikely to wait that long.