Vance signalled that he was still giving time to Iran to consider the offer from the United States, which on Tuesday said it would pause attacks with Israel for two weeks pending negotiations.

"We leave here with a very simple proposal, a method of understanding that is our final and best offer. We'll see if the Iranians accept it," Vance told reporters after 21 hours of talks in the Pakistani capital Islamabad.

Vance said that the core dispute was on nuclear weapons. Iran insists it is not pursuing an atomic bomb, and the United States and Israel bombed sensitive Iranian sites both in the war launched on February 28 as well as last year.

"The simple fact is that we need to see an affirmative commitment that they will not seek a nuclear weapon and they will not seek the tools that would enable them to quickly achieve a nuclear weapon," Vance said.

"The simple question is, do we see a fundamental commitment of will for the Iranians not to develop a nuclear weapon -- not just now, not just two years from now, but for the long term?

"We haven't seen that yet. We hope that we will."

Vance, in brief remarks at a luxury hotel in Islamabad where the two sides have been meeting, did not highlight disagreement on another key issue, the reopening of the Strait of Hormuz, the narrow passageway through which one-fifth of the world's oil transits.

He insisted that President Donald Trump -- who on Saturday in Washington said he did not care if the two sides sealed a deal -- had been accommodating in the talks.

"I think that we were quite flexible. We were quite accommodating. The president told us, You need to come here in good faith and make your best effort to get a deal.

"We did that and, unfortunately, we weren't able to make headway."