WASHINGTON: Negotiators have made significant progress in the last week or so on an agreement to revive the 2015 Iran nuclear deal but very tough issues remain, a senior US State Department official said on Friday.

“There’s been significant progress over the last week or two. We have significantly narrowed the areas that still need to be resolved. And so, in that sense, there has been progress that has been moving towards a potential deal,” he told reporters.

“So we are in a better position than we have been. But at the same time it’s important to note that very serious issues remain.”

The aim of the negotiations is to return to the original 2015 bargain of lifting sanctions against Iran, including ones that have slashed its oil sales, in exchange for restrictions on its nuclear activities that extend the time Tehran would need to make enough enriched uranium for an atomic bomb if it chose to.