WASHINGTON - Describing US-Pakistan relationship as "complicated", a State Department spokesman has said that the two countries were working to deal with the issues between them.

"It is an important, vital relationship that we strongly believe in," spokesman John Kirby told reporters at the daily press briefing on Friday.

He was responding to a question about Pakistani Foreign Affairs Adviser Sartaj Aziz's statement that Washington-Islamabad ties were under "stress" for the past three months because of conditions Washington had attached to the funding of F-16 fighter jets sale.

Kirby said he had not seen Sartaz Aziz's comments, but added that the US values its relationship with Pakistan. "Is it complicated at times", the spokesman posed the question and then himself answered, "Absolutely, it is."

"And do we see eye-to-eye on every issue with Pakistan? No, we don’t. But that’s why the relationship matters so much, because we have shared threats and shared concerns, shared interest in the region, and we’re going to continue to work at it," Kirby said.

Besides the funding of F-16s, the State Department has also said that key Congress members were not prepared to support military aid to Pakistan. “I will note key members of Congress have been clear they’re not prepared to support US military aid to Pakistan absent some specific actions,” Elizabeth Trudeau, Director Office of Press Relations State Department said at a daily press briefing on Thursday.

Kirby side-stepped a question about Afghan President Ashraf Ghani's statement that his country was fighting an undeclared war with Pakistan, saying the two neighbours still faced a shared threat from terrorist networks which continue to use the spine between those two countries as safe haven.

"That’s why we still have a counterterrorism presence in Afghanistan," he said. "It’s why we continue to work with the Government of Pakistan as best we can to help share information as appropriate to help all sides go after this shared threat. This is a shared, common enemy to the people of Afghanistan and to the people of Pakistan, and they have been working and communicating together, and we want to see that kind of dialogue and cooperation continue and to improve."

About the closure of Pak-Afghan border at Torkham, Kirby noted that the two countries on Friday agreed to reopen Torkham border crossing for vehicles coming in and going out, and resumption of routine activities.

The decision came in a meeting between Afghan Ambassador Omer Zakhilwal and the Chief of Staff of the Pakistan Army, General Raheel Sharif.

Responding to a question about US stance regarding India’s membership of Nuclear Suppliers Group (NSG), the spokesman said President Barack Obama reaffirmed US views on the issue during his last year visit to India that New Delhi meets missile technology control regime requirements and is ready for NSG membership.

His remarks came in response to a question on reports that China and Pakistan have joined hands to oppose India becoming a member of the NSG.

"Deliberations about the prospects of new members joining the Nuclear Suppliers Group are an internal matter among current members," Mr Kirby added.