Vance's comments came hours after President Donald Trump said he had come within an hour of ordering fresh strikes and gave Tehran "two or three days" to reach an agreement.

"A lot of good progress is being made, but we're just going to keep on working at it, and eventually we'll either hit a deal or we won't," Vance told a briefing at the White House.

Vance -- an Iran war-skeptic who led a US delegation to Pakistan in April for talks that failed to produce a deal -- said Iran had to accept that it could not have a nuclear weapon.

"We're in a pretty good spot here -- but there's an option B, and the option B is that we could restart the military campaign," said the US vice president.

"We're locked and loaded. We don't want to go down that pathway, but the president is willing and able to go down that pathway if we have to."