The measured Afghanistan endgame that President Obama outlined this week suffered new setbacks Thursday, as the Taliban suspended peace talks with the United States and Afghan President Hamid Karzai demanded that NATO withdraw forces from the small, rural outposts that are at the heart of its military mission here.

The developments are the latest in a cascade of challenges to the exit that the administration and its coalition partners are planning, including a gradual turnover of security responsibility to Afghan troops, a paced U.S. and NATO withdrawal and a negotiated peace with the Taliban.

Administration officials had steeled themselves for fallout after the burning of Korans by U.S. service members last month and the massacre of 16 Afghan civilians Sunday, allegedly by an Army staff sergeant who went on a rampage.

But Thursday’s statements caught Western officials in Kabul off guard and sparked new concerns that the U.S.-led operation here could unravel as trust erodes between Afghans and their foreign benefactors.

“Afghanistan is ready right now to take all security responsibilities completely,” Karzai said in a statement issued shortly after he met with visiting Defense Secretary Leon E. Panetta. “To speed up this process, authority should be given to Afghans.”

The statement set no deadline for what it called the “withdrawal of international forces from villages.” It reiterated Karzai’s insistence that foreign troops should not be allowed to conduct night raids on Afghan homes. U.S. commanders rely heavily on these operations to net suspected insurgents.

Karzai has a long history of making demands that the international community ignores or implements slowly, and he has cried wolf many times. But if he presses ahead with the demand for withdrawal of NATO forces from the countryside, the U.S. military could face many of the same challenges it contended with in Iraq in 2009, as Baghdad sought to curb the movement and authority of American troops.

The effect in Afghanistan could be considerably more complex and dangerous. Iraq’s insurgency had begun to wane at the time, but militants in Afghanistan remain strong, despite coalition progress in some areas, and Taliban leaders continue to have sanctuary in neighboring Pakistan.

Coalition commanders see the constellation of small outposts in insurgent-plagued provinces as essential to their goal of providing enough security for the Afghan government to take root. That objective remains elusive in much of the country, and the Afghan government, whose military forces are still a work in progress, still depends almost entirely on foreign funding.

In a news conference in Washington with visiting British Prime Minister David Cameron on Wednesday, Obama acknowledged “multiple challenges along the way” but said that “in terms of pace, I don’t anticipate at this stage that we’re going to be making any sudden additional changes to the plan that we currently have.”

“If we maintain a steady, responsible transition process, which is what we’ve designed, then I am confident that we can put Afghans in a position where they can deal with their own security,” he said.

The plan NATO adopted in late 2010 has already undergone some changes. Although it had indicated that the turnover of security responsibility to the Afghans would not be completed until 2014, with NATO combat forces withdrawing by the end of that year, the alliance now hopes to transition to a supporting role throughout the country by the end of next year.

Karzai’s statement said the transition should be completed in 2013, and it was unclear whether he was proposing a game plan different from NATO’s.

Obama also said that he expected a U.S.-Afghanistan strategic partnership agreement to be completed by the next NATO summit, in Chicago in May. But that agreement, designed to set the terms for a reduced but continued U.S. military presence in Afghanistan after 2014, has foundered over the issue of night raids.

Karzai has “just generally always been concerned that night raids are intrusive and heavy-handed, whereas what we’ve pointed out is that they’re central to our ability to degrade Taliban operations and battlefield leadership,” said a senior administration official in Washington, speaking on the condition of anonymity to discuss the sensitive negotiations. “We’re trying to find the appropriate course of action.”

Administration officials played down the significance of the Taliban statement suspending peace talks, describing it as part of a roller coaster they have grown familiar with ever since a series of U.S. meetings with the insurgents, most of them in the Persian Gulf emirate of Qatar, began in late 2010.

But the talks have been stalled for nearly two months, and the new announcement further diminished U.S. hopes that progress could be announced at the NATO meeting as planned.

Late last year, agreement was reached between U.S. officials and Tayeb al-Agha, an aide to Taliban leader Mohammad Omar, on the exchange of five Taliban suspects being held at the U.S. prison at Guantanamo Bay, Cuba, for a Western official held by the insurgents. The Washington Post has withheld the name of the official for safety reasons, at the administration’s request, since the fall.

Other elements of the tentative deal included what U.S. officials said would be a Taliban statement recognizing the authority of the Afghan government and renouncing international terrorism, and the establishment of a Taliban office in Qatar. But the plan faltered when Karzai objected, saying he had not been consulted. He later suspended ties with Qatar, where Taliban negotiators had established a beachhead.

By January, Karzai reversed course, and U.S. officials said the opening of the Qatar office would also mark the moment when the Afghan government would enter the talks. Congress was briefed on the exchange, under which an initial three Taliban prisoners were to be transferred to house arrest in Qatar, with the other two following 60 days later.

Administration optimism rose when an Afghan government delegation visited the Guantanamo prisoners last week and when Karzai announced Saturday that he would send a senior representative to Qatar for talks with that government. But spirits fell again Sunday at news of the massacre of the Afghan civilians.

In the Thursday statement, posted on its Web site, the Taliban said it was suspending all talks “until the Americans clarify their stance.” It accused the United States of breaking its promise on the prisoner exchange and imposing a new “list of conditions” — an apparent reference to the statement on international terrorism that the insurgents have yet to make.

It also reiterated that Taliban would talk only to the “American invaders” and not to the Karzai government.

“Now they say it’s the Americans who are the problem,” said a second administration official, referring to the announcement. “It was too much to expect that nothing would happen” in the wake of Sunday’s incident, he added.

DeYoung reported from Washington. Staff writer Greg Jaffe in Kabul contributed to this report.