CHICAGO â€"  U.S. tensions with Pakistan complicated the opening day of the NATO summit in Chicago, where allies gathered to solidify a strategy for ending the war in Afghanistan.

The Obama administration has so far been unable to reach an agreement with Pakistan to reopen key supply routes into Afghanistan that were closed after a Nov. 26 U.S. strike on two border posts that killed two dozen Pakistani troops.

U.S. and NATO officials intend to use the summit to intensify pressure on Pakistani President Asif Ali Zardari to cut a deal to reopen the supply routes, though hopes for an immediate breakthrough have faded, The Wall Street Journal reported.

Zardari was invited to attend the two-day summit at the last minute in hopes that would lead to a deal, but the two sides remain at odds over how much the U.S. and its allies should pay Pakistan to move cargo through the route.

FOX News Channel reported that US Secretary of State Hillary Clinton met with Zardari, with a State Department spokesperson saying, "They discussed the importance of reopening the NATO supply lines" and of addressing terrorist threats from "Al Qaeda and the Haqqani Network."

"The United States is committed to a strong, mutually beneficial relationship built on concrete actions to enhance the security and prosperity of Pakistan, the United States and the region," the spokesperson said.

The impasse quickly became an undercurrent of the summit, which already was confronting Obama with an election-year balancing act: rallying NATO leaders around a plan to remain engaged in Afghanistan through 2024 and spotlighting his commitment to withdraw U.S. troops from a war that has grown deeply unpopular at home, the Journal reported.

Obama stressed Sunday that under the U.S. and NATO allies' plan, within the next two years "the Afghan war, as we understand it, is over." At the same time, speaking after meeting with Afghan President Hamid Karzai, Obama said summit attendees are "also painting a vision" for Afghanistan over the long-term, one in which the U.S. will have a central role.

Karzai, in turn, said Afghanistan is looking forward to the time when it "is no longer a burden" to the international community and thanked the U.S. for the "support of your taxpayers' money" since the war began in the fall of 2001.

John Allen, the commander of U.S. and NATO forces in Afghanistan, played down the impact of the closure of Pakistan's border crossings on the day-to-day military campaign. He said the U.S. does not know when a deal might be reached, but "sooner is better than later."

But according to U.S. officials, Pakistan has proposed raising transit fees per container by as much as 3,000 percent or 30-fold, a demand that Washington and its allies have rejected as excessive.

Pakistan's closure of the supply routes has forced the U.S. to expand an alternative northern distribution network, which winds its way from Baltic and Caspian ports through Russia, Central Asia and the Caucasus.

Transporting a container to Afghanistan through the northern network costs more than 21 times as much as it would to bring the same container through Pakistan before the supply routes were closed.

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