By Missy Ryan and David Brunnstrom
CHICAGO (Reuters) - Pakistani President Asif Ali Zardari arrived in Chicago on Saturday for a NATO summit to what could be a chilly reception if a hoped-for deal allowing trucks to again supply alliance troops in Afghanistan fails to materialize.
While Western officials sought to portray Zardari's presence as a sign of improving NATO-Pakistan ties, possible friction at the meeting underscores the challenges NATO countries face as they struggle to ensure a stable future for Afghanistan after Western troops withdraw.
Western leaders, who sometimes differ in their visions for the best way to end the war, speak with one voice when they describe Pakistan as the lynchpin to Afghanistan's future security - or continued conflict.
"We can't solve the problems in Afghanistan without a positive engagement of Pakistan," NATO Secretary-General Anders Fogh Rasmussen said hours before Zardari's arrival, repeating a refrain from NATO leaders that Pakistan must do more to act against militants who launch attacks from the country's tribal areas.
Zardari's decision to accept a last-minute invitation to attend the summit in President Barack Obama's home town was an abrupt reversal after months of diplomatic estrangement. Islamabad had made clear that Obama's top aides were not welcome in Pakistan.
It was also initially seen as a sign that Islamabad was on the verge of reopening supply routes through Pakistan that are crucial for NATO troops in Afghanistan.
But now it is far from clear that a deal on those supply lines is imminent.
"It is time for them to make a decision," said a U.S. official. "Patience is not exhausted but it's wearing thinner on the U.S. side."
In a sign of U.S. frustration, the White House said this week that Obama had no plans to meet with Zardari individually, although he is scheduled to meet with Afghan President Hamid Karzai.
A RIFT NOT YET HEALED
Pakistan closed the supply routes in November after U.S. aircraft killed 24 of its soldiers along the border with Afghanistan. The incident caused a rift with the United States that has not yet healed as the White House, gearing up for November elections, has rebuffed Pakistan's demands for an apology.
The U.S. official, speaking on condition of anonymity, said there have been no breakthroughs in talks on the supply routes that many in the Obama administration had hoped would yield an agreement at the summit, which begins on Sunday.
Yet U.S. and NATO officials said the mere fact that Zardari would attend is a step forward.
"Obviously it's important that they are taking part in the summit as it's a big show of support for Afghanistan," said NATO spokeswoman Oana Lungescu.
The Obama administration is eager to resume more productive ties with Pakistan, whose role will be even more crucial as NATO combat troops begin to withdraw ahead of a 2014 deadline.
"The goal is to re-establish a sense of normalcy in the relationship and we're getting there. It will take time," the U.S. official said.
The Obama administration accuses Pakistan of sheltering militants who attack Karzai's government and NATO troops. Pakistan sees Afghanistan as vital to its security concerns, and wants a say in the country's future.
Islamabad defends its role in combating militants and notes that many of its own soldiers have died in that fight.
"The point here is that Pakistan has cooperated in every effort to hunt down, degrade, destroy and disable al Qaeda. And your military acknowledges that," Sherry Rehman, Pakistani ambassador to the United States, told CNN on Friday.
Zardari's government has voiced its willingness to reopen the supply routes - if an agreement can be reached on fees that will be paid on each shipment going into Afghanistan.
In an interview with the Chicago Tribune, Defense Secretary Leon Panetta said it was unlikely the United States would accept Pakistani requests to pay as much as $5,000 per military shipment going into Afghanistan.
(Additional reporting by Phil Stewart. Editing by Christopher Wilson)