Washington: The US has said its ties with Pakistan are a "vital relationship" that it strongly believes in, but acknowledged that both don't see eye to eye on every issue, amid strain in ties with Islamabad over blocking the sale of F-16s. "It is a important, vital relationship that we strongly believe in," State Department spokesman John Kirby said on Friday during his daily news briefing. "Is it complicated at times? Absolutely, it is. And do we see eye-to-eye on every issue with Pakistan? No, we don’t," Kirby added. Kirby's comments come after Pakistan Prime Minister's Foreign Affairs Adviser Sartaj Aziz on Thursday conceded that relations with the US have been under stress for the past three months because of conditions Washington has attached to the funding of F-16 fighter jets sale. Briefing the Pakistani senate, Aziz said on Thursday that the upward trajectory of ties with Washington has "witnessed a downward slide" in the past three months due to the "decision of the US Congress to block partial funding for 8 F-16 aircraft through the use of FMF (Foreign Military Financing)". The US had said Pakistan would have to pay in full if it wants to go through with the nearly $700 million deal for the F-16s. Pakistan was to get a $429 million subsidy for the jets. Pakistan has said it will consider other options if it cannot buy the F-16s. Kirby added reassuringly, "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."