WARSAW: Defense Secretary Chuck Hagel hinted Thursday at growing US impatience with Afghan President Hamid Karzai for his refusal to sign an accord permitting American troops to remain in his country after the US combat mission ends in December.

Hagel told reporters that at some point Karzai's indecision will interfere with Washington's need to plan the post-2014 military mission that Karzai himself has said he favors.

The Obama administration has indicated it might be willing to keep as many as 10,000 U.S. troops in Afghanistan for some time after 2014 to train and advise Afghan forces.

Hagel pointedly noted that Karzai ''agreed, personally agreed, to the bilateral security agreement'' that was negotiated between the two nations last year, yet has balked at signing it.

''You can't just keep deferring and deferring, because at some point the realities of planning and budgeting, it collides,'' Hagel said aboard his plane on an overnight flight from Washington.

Later Thursday, Karzai's national security adviser said he is more hopeful that the Afghan leader will sign the agreement before leaving office this year, although the Obama administration repeatedly has said since the new year that the agreement must be signed in ''weeks, not months.''

In Kabul, Rangin Dadfar Spanta said intense talks in the last few days have made him ''more optimistic'' that the deadlock can be broken. Asked about those comments at a news conference in Warsaw, Hagel said it's difficult to know what to believe.

''What is coming out of the presidential palace today, or what President Karzai says today, I don't know,'' he said. ''It changes constantly.''

Hagel said US officials, including Gen. Joseph Dunford, the top US military commander in Afghanistan, have pressed Karzai and ''talk with him constantly.''

The Pentagon chief said he respects Karzai's right to decide the matter as he sees fit, and noted that the United States' ability to influence Karzai's decision-making is ''limited.''

He added that US allies who are willing to help train and advise Afghan forces beyond 2014 also are eager to know if there will be a US-Afghan security agreement soon.

The US now has about 39,000 troops in Afghanistan but would reduce that figure to zero by year's end unless a security accord is signed in the months ahead.

Hagel was visiting Warsaw to consult with Polish officials on Afghanistan and other security issues.