CHICAGO - President Obama and Pakistani President Asif Ali Zardari met briefly today on the sidelines of the NATO summit, but made no progress toward resolving their diplomatic standoff over supply routes into Afghanistan.

Their meeting, which was also attended by Afghan President Hamid Karzai, comes amid mounting tension over Pakistan's refusal to re-open its Afghanistan border to NATO, forcing the United States and others to supply their forces through longer, alternative routes. The channels would also be crucial to NATO's exit plan to withdraw forces and military equipment from Afghanistan, posing a potential logistical nightmare for the alliance.

"President Zardari shared with me his belief that these issues can get worked through," Obama later told reporters at a press conference. "We didn't anticipate that the supply line issue was going to be resolved by this summit. We knew that before we arrived in Chicago. But we're actually making diligent progress on it.

"I think ultimately, everybody in the alliance, all of ISAF and, most importantly, the people of Afghanistan and Pakistan understand that neither country is going to have the kind of security, stability and prosperity that it needs unless they can resolve some of these outstanding issues and join in common purpose with the international community in making sure that these regions are not harboring extremists," Obama added.

Pakistan closed the routes after two dozen Pakistani soldiers were killed in a U.S. airstrike last November.

Relations between the U.S. and Pakistan have been deteriorating since Obama ordered the secret raid that killed Osama bin Laden in his Pakistani compound last year.

"I don't want to paper over real challenges there," Obama said. "There's no doubt that there have been tensions between ISAF and Pakistan, the United States and Pakistan over the last several months… But ultimately, it is in our interest to see a successful, stable Pakistan, and it is in Pakistan's interests to work with us and the world community to ensure that they themselves are not consumed by extremism that is in their midst. And so we're going to keep on going at this."

The three presidents met after a larger meeting with NATO leaders on the transition in Afghanistan. Obama also had a brief one-on-one conversation with Zardari as they made their way into the meeting earlier this morning, according to the White House.