But American officials say they need a deal finalized soon, in part to give European allies, who lack the robust logistics capability of the United States military, time to plan for an extended mission in Afghanistan. They also said the longer they waited, the more likely those in the administration — and the public — who want a complete withdrawal would gain support.

“We’ve been clear that our preference is to conclude the B.S.A. by the end of the year, and that if we cannot conclude a B.S.A. promptly thereafter, then we will be forced to initiate planning for a post-2014 future in which there would be no U.S. or NATO troop presence in Afghanistan,” said Laura Magnuson, a spokeswoman for the White House. “That has not changed.”

With the deal in limbo, the official focus in Kabul has shifted to talks on a companion agreement that would allow other NATO members, such as Italy and Germany, to keep forces in Afghanistan after the alliance’s combat mission formally ends next year.

The NATO-led coalition here announced the start of talks in a statement this past weekend. The statement was carefully worded, Western officials said, to head off any talk from Mr. Karzai of cutting out the Americans and trying to work only with European allies, an idea he has previously floated in meetings with Western officials.

“The NATO status of forces agreement will not be concluded or signed until the signature of the bilateral security agreement between the governments of Afghanistan and the United States,” the NATO secretary general, Anders Fogh Rasmussen, said in the statement.

But Mr. Karzai appears to still be holding out for a better deal with the United States, despite the original accord’s endorsement last month by a grand council of Afghans, known as a loya jirga, and economic signs that public confidence is falling in the absence of an agreement.

“I want this security agreement with the U.S.,” a statement from the presidential palace quoted Mr. Karzai as saying in an off-the-record meeting with Afghan journalists on Sunday. “But Afghans’ homes should be protected from American operations, and Afghanistan should not become the battleground of a continuous war.”