President Obama shows signs of frustration with Pakistan A growing diplomatic dispute over Pakistan's refusal to reopen supply routes for U.S. and other troops in Afghanistan threatened to overshadow President Obama's efforts on the final day of the NATO summit to finalize plans for ending the unpopular war. In remarks to more than 50 world leaders, Obama on Monday made clear his frustration at what aides view as Pakistani intransigence by publicly thanking Russia and other Central Asian countries for providing "critical transit" of war supplies into Afghanistan in the six months since Pakistan closed its ground routes following U.S. airstrikes that killed 24 Pakistani soldiers. Pakistani President Asif Ali Zardari sat only a few feet away, but Obama pointedly did not mention Pakistan and initially refused to meet him one-on-one, part of a campaign aimed at persuading Islamabad to abandon its demands for a formal apology and for steeply higher payments in return for resuming the shipments. In a news conference later, Obama made no attempt to downplay the problems in a relationship with a nominal ally that has spiraled from one crisis to another since U.S. Navy SEALs flew into the military garrison town of Abbottabad last May to kill Osama bin Laden, infuriating many Pakistanis. "We need to work through some of the tensions" with Pakistan, Obama said. I don't want to paper over real challenges. Obama also offered a public window into how his thinking about the use of military force had developed, reflecting on an issue that officials have said increasingly has been on his mind -- the inevitable tensions that develop when the U.S. tries to keep a large military force operating actively in a foreign country for a lengthy period. "No matter how much good we're doing," Obama said, the sort of military "footprint" the U.S. has maintained in Afghanistan inevitably causes tensions that "over time can be counter-productive." "Ten years in a county that's very different, that's a strain," he said. Early in his tenure, Obama had authorized a large increase in the U.S. troop presence in Afghanistan. Last year, however, he set the United States on a path to withdraw nearly all U.S. forces by the end of 2014, overruling military figures who had sought a longer troop commitment. As he has said previously, Obama stressed that one reason he believed it was wise to set a deadline is that Afghanistan's government needs one. Afghan security forces "will not ever be prepared if they don't start taking that responsibility" to protect their own country's security, he said. But in Monday's remarks, Obama not only talked openly about the limits he sees in how long a U.S. military presence can be sustained. He also suggested that he had applied that lesson in setting ground rules for U.S. forces in Yemen, Somalia, Mali and other places in which they have battled groups affiliated with Al Qaeda. The goal there, he said, is to "stay focused on the counter-terrorism issue, to not over-extend ourselves." On the strains with Pakistan, Obama said his staff members had known before they arrived in Chicago on Saturday that they would not resolve the dispute over access to supply lines, and he gave no prediction of how quickly they might reach a solution. "We're making diligent progress on it," he said. U.S. officials acknowledged privately that NATO had invited Zardari to the summit at the last minute, hoping a high-profile meeting with Obama might provide an incentive for a deal on resuming supply shipments. When that strategy did not work, they tried to raise the pressure through what appeared a series of carefully calibrated slights. "The invitation was an inducement to get them back into the international fold," said a senior U.S. official, speaking anonymously because of the sensitive issues involved. But the Pakistanis couldn't get their own act together. Some U.S. officials said that the public snubs by the White House could backfire by undermining Zardari at home, leaving him politically weaker and even less able to overcome the intense anti-U.S. feeling in Pakistan over the bin Laden raid, the Nov. 26 attack on the border posts, and years of lethal CIA drone attacks on Pakistani territory. To lower the tensions, the White House announced Monday afternoon that Obama had held a brief meeting with Zardari and Afghan President Hamid Karzai, and rushed out a picture of them together. But in his news conference, Obama did not describe the discussion in either warm or positive terms. "It was very brief as we were walking into the summit," he said.