Washington, May 22 (IANS): The US continues to negotiate with Pakistan towards a resolution that will lead to the reopening of NATO supply routes that have been closed since November, US Defense Secretary Leon Panetta has said.

It was a positive sign that Pakistani President Asif Ali Zardari attended the NATO summit held in Chicago, Geo News quoted Panetta as saying in the US city.

"We still have a way to go, but I think the good news is that we are negotiating and that we are making some progress," he said.

"It is extremely important that ultimately we're able to open up those lines of communication and transport so that we can expedite the assistance that needs to go to our men and women in uniform who are fighting the battle," the US Department of Defense website quoted Panetta as saying.

The Pakistan-US relationship suffered a strain when a cross-border air strike by NATO forces at a Pakistan Army border post killed 24 soldiers in November.

"I guess I would say that I feel a lot more positive about the effort to try to see if we can find a resolution to that challenge," Panetta said.

The US official said that another continuing challenge associated with Pakistan and Afghanistan involved the Taliban.

"I think we understand that the biggest challenge is a Taliban that is resilient, that is going to continue to fight even though they've been weakened -- and I think the levels of violence are down - and that they're going to continue to conduct attacks," he said.

NATO troops and Afghan forces will have to confront that enemy, Panetta said, adding that he has confidence that the Afghan army can respond effectively and be part of that effort.

"We are still dealing with a resilient enemy that in many ways still has a safe haven in Pakistan. And that, I think, represents the greatest threat that we're facing," he said.