American officials say they are preparing to suspend negotiations absent a breakthrough in the coming weeks, and a senior administration official said talk of resuming them with President Hamid Karzai’s successor, who will be chosen in elections set for next April, is, “frankly, not very likely.”…

The impasse, after a year of talks, has increased the prospect of what the Americans call the zero option — complete withdrawal — when the NATO combat mission concludes at the end of 2014. That is precisely the outcome they hoped to avoid in Afghanistan, after having engaged in a similarly problematic withdrawal from Iraq two years ago.

Moreover, a complete withdrawal from Afghanistan could be far costlier than it was in Iraq. It would force European powers to pull their forces as well, risking a dangerous collapse in confidence among Afghans and giving a boost to the Taliban, which remain a potent threat…

Many contentious matters in the talks have already been settled, like legal immunity for American troops, which is what scuttled the Iraq deal, Afghan and American officials said. Yet officials on both sides say two seemingly intractable issues remain.

The first is Afghanistan’s insistence that the United States guarantee its security, much like any NATO ally, and the second is Mr. Karzai’s refusal to allow American forces to keep searching in Afghanistan for operatives of Al Qaeda. Instead, he has proposed that the United States give its intelligence information to Afghan forces and let them do the searching, said Aimal Faizi, a spokesman for the president.