ISLAMABAD, Pakistan — The latest high-level talks on ending a diplomatic deadlock between the United States and Pakistan ended in failure on Friday over Pakistani demands for an unconditional apology from the Obama administration for an airstrike. The White House, angered by the recent spectacular Taliban attacks in Afghanistan, refuses to apologize.

The Obama administration’s special envoy to Afghanistan and Pakistan, Marc Grossman, left the Pakistani capital Friday night with no agreement after two days of discussions aimed at patching up the damage caused by the American airstrikes last November that killed 24 Pakistani soldiers on the Afghanistan border.

Both sides insist that they are now ready to make up and restore an uneasy alliance that at its best offers support for American efforts in Afghanistan as well as the battle against some extremist groups operating from Pakistan. The administration had been seriously debating whether to say “I’m sorry” to the Pakistanis’ satisfaction — until April 15, when multiple, simultaneous attacks struck Kabul and other Afghan cities.

“What changed was the 15th of April,” said a senior administration official.

American military and intelligence officials concluded the attacks came at the direction of a group working from a base in North Waziristan in Pakistan’s tribal belt: the Haqqani network, an association of border criminals and smugglers that has mounted lethal attacks on foreign forces in Afghanistan. That confirmed longstanding American mistrust about Pakistani intentions — a poison that infects nearly every other aspect of the strained relationship. That swung the raging debate on whether Mr. Obama or another senior American should go beyond the expression of regret that the administration had already given, and apologize.

The negotiations are complicated by a complex web of interlocking demands from both sides. Without the apology, Pakistani officials say they cannot reopen NATO supply routes into Afghanistan that have been closed since November.

The Americans, in turn, are withholding between $1.18 billion and $3 billion of promised military aid — the exact figure depending on which side is speaking.

The continuing deadlock does not bode well for Pakistan’s attendance at a NATO meeting in Chicago in three weeks, assuming it is even invited. The administration has been eager to cast the event as a regional security summit meeting, and Pakistan’s absence would be embarrassing.

Administration officials acknowledged Friday that the stalemate would not be resolved quickly. “This is the beginning of the re-engagement conversation,” Victoria Nuland, the State Department spokeswoman, said in Washington. “We’re going to have to work through these issues, and it’s going to take some time.”

The two countries at least are relieved to have started talking. A series of visits and discussions in recent weeks included a meeting between Mr. Obama and Prime Minister Yousaf Raza Gilani on the sidelines of a nuclear summit meeting in Seoul, South Korea, last month. Since the Pakistani Parliament completed a review of relations with the United States, Americans have repeatedly vowed to respect the will of Pakistan’s lawmakers, even though they demanded an end to American drone strikes, which the United States sees as crucial in fighting militants hiding in Pakistan’s border areas.

Aside from the apparently intractable issues of drones and the apology, the two countries focused on four specific areas of potential cooperation: counterterrorism, the NATO supply lines, military aid payments and the Taliban peace process.

Yet there was an undeniable sense of wariness, driven by the pressures of domestic politics, with Mr. Obama facing re-election this year and Pakistan due for elections in the coming 12 months. Pakistanis’ rage has been rising since a shooting in Lahore in January 2011 that involved a C.I.A. employee and fueled common fantasies about being overrun by rogue spies. The American operation to kill Osama bin Laden a few months later was taken as a stunning breach of Pakistan’s sovereignty.

An American apology is also problematic given Republican pressures weighing on Mr. Obama and the hostility of a Congress with little patience for Pakistan. “The politics of election year in both countries are slowing down the resolution of admittedly vexed issues in an environment of persistent mistrust,” said Maleeha Lodhi, a former Pakistani ambassador to Washington.

The Haqqani network has re-emerged as a focal American issue, particularly after the April 15 attacks. The next day, Secretary of State Hillary Rodham Clinton told Pakistan’s foreign minister, Hina Rabbani Khar, that “there has to be a concerted effort by the Pakistanis with the Afghans, with the others of us, against extremists of all kinds.”

American officials refused Friday to say whether there were any links between Pakistan’s main spy agency, the Inter-Services Intelligence Directorate, or ISI, and the Haqqani network’s latest attacks. One said the intelligence on the issue was “constantly evolving.” Others in Washington say they have not yet found any such ties.

New details about the attacks have emerged in the past two weeks, according to Afghan and American officials. While it is possible that some fighters were smuggled into Afghanistan over time and in small numbers, and that some weapons and ammunition were pre-staged, many may have been brought in from Pakistan only a day or two before the attacks, said a senior American military officer in Afghanistan.

“Our initial assessment is they probably moved them in a last moment to avoid detection,” said the officer, who spoke on the condition of anonymity because of the continuing inquiry.

Officials have also identified a possible intelligence gap. Ethnic infighting at the top of Afghanistan’s intelligence agency, the National Directorate of Security, may have resulted in key people failing to pass on information that could have helped derail the attacks.

At this week’s meetings in Islamabad, new ideas were gently sounded out.

A senior Pakistani official said his country was offering a “wide menu of counterterrorism options” in a bid to at least slow down the rate of drone strikes. Pakistan has also offered to send F-16 fighter jets to strike Taliban and Qaeda targets in the tribal belt.

United States officials have said that if Pakistan would not or could not strike insurgents in places like Miram Shah, the capital of North Waziristan, then the drone attacks would have to continue. With Pakistan refusing at least publicly to condone the strikes, the two sides seem at an impasse.

“The policy of the government is very, very clear,” Pakistan’s foreign secretary, Jalil Abbas Jilani, said Thursday. “We consider drones as illegal, counterproductive and, accordingly, unacceptable.”

Another Pakistani official, however, conceded, “Privately, we know they are unlikely to stop.”

The reopening of NATO supply lines is important for the United States military to support troops currently in Afghanistan, but also to help withdraw tons of weapons and matériel out as a major drawdown approaches in 2014. But, the senior Obama administration official added, Pakistan’s support for the NATO lines was about politics as much as logistics. “Our NATO partners see them as increasingly problematic, not as a partner,” he said. “If they don’t restore this, those feelings will become intensified over time.”

Declan Walsh reported from Islamabad, and Eric Schmitt and Steven Lee Myers from Washington. Alissa J. Rubin contributed reporting from Kabul, Afghanistan. (Copyright 2012 The New York Times Company 04/28/12)