A summit meant to symbolize a thaw between the United States and Pakistan has only worsened the bad blood, with the troubled relationship casting a pall over NATO plans on Afghanistan.

The Western alliance at the last minute invited President Asif Ali Zardari to a summit in Chicago on the future of Afghanistan, with officials predicting a deal with Pakistan on reopening supply routes vital for US and allied troops.

But no deal materialized, in an apparent dispute over how much to pay Pakistan. Zardari left Chicago jilted after President Barack Obama only agreed to see him in passing, while US officials increasingly voiced exasperation on finding a way forward with Pakistan.

Obama, speaking Monday at the end of the conference, admitted frustration, saying: "There's no doubt that there have been tensions between (NATO forces) and Pakistan, the United States and Pakistan over the last several months."

But Obama said that US priorities remained the same -- that "Pakistan has to be part of the solution in Afghanistan, that it is in our national interest to see a Pakistan that is democratic, that is prosperous and that is stable."

Obama, soon after his election, determined to put a new focus on Pakistan amid US concerns that part of the country's powerful military and intelligence still supports Islamic extremists after a decade of war in Afghanistan.

Obama has stepped up drone attacks deep into Pakistani territory but has also hoped to empower civilian institutions. Zardari took office in 2008 after a decade of military-backed rule and, despite deep unpopularity, is on course to be the first democratic Pakistani leader to complete a full term.

Bruce Riedel, who led the administration's initial strategy review on Afghanistan and Pakistan after Obama took office in 2009, criticized the US decision not to hold a full-fledged meeting with Zardari in Chicago.

"Maybe I'm old-fashioned, but that's bad form," said Riedel, a former US policymaker who is a senior fellow at the Brookings Institution.

"Zardari is an extremely proud person and to be snubbed in the way that he was, I think is going to come home to haunt us," Riedel said.

While not uncritical of Zardari, Riedel said that the president was the Pakistani power-broker most in tune with the US vision for the region.

Zardari -- whose wife, former prime minister Benazir Bhutto, was assassinated in 2007 -- has called on Pakistan to do more to battle Islamic extremists and worked to ease tensions with India, last month paying a rare trip to the historic rival.

"I hope that what happened in Chicago won't discourage him, but I have a bad feeling that this is a very significant setback for us," Riedel said.

For Pakistanis, the Chicago snub could reinforce their long-held suspicions -- that the United States will abandon their country once it completes its withdrawal of troops from Afghanistan in 2014.

Some Pakistani analysts have cited the specter of a US departure to justify maintaining contact with hardliners in Afghanistan. The United States has accused Pakistani intelligence of close coordination with the Haqqani network, which is blamed for attacks on US troops.

Pakistan, which helped create Afghanistan's Taliban regime, threw its support behind the United States after the September 11, 2001 attacks.

But it shut down its supply routes on the border after US air strikes killed 26 Pakistani troops in November on the border. Obama has voiced regret, calling the deaths an accident, but has refused demands for an apology.

US officials said that plans to reopen the supply routes broke down after Pakistan dramatically raised its demands on payment for each truck going through the border.

The United States has provided Pakistan more than $18 billion in assistance since 2001 and the aid is highly sensitive. A number of lawmakers have sought to end assistance after US forces killed Osama bin Laden in Pakistan last year, raising allegations that intelligence services may have abetted the world's most wanted man.

Some US officials suspected privately that Zardari was being undercut in Chicago by the military, which wanted full control of the war effort and to deprive the civilian leader of a success.

Lisa Curtis, a senior research fellow at the Heritage Foundation, a conservative think tank, said there was no "personal snub" to Zardari.

"It was simply making a statement that Pakistan is not helping with the Afghanistan effort. If Pakistan wants to help, it will be welcome at the table, but so long as it remains outside that effort and is in fact undermining that effort, then Pakistan should be snubbed," she said.