Iran’s stockpile of enriched uranium is believed by Western negotiators and international inspectors to be of far lower purity than is required to make nuclear weapons. But, diplomats in Vienna said on Tuesday, enriched uranium converted into reactor fuel is more difficult to enrich to a higher degree of purity. “It’s a step away from weaponization,” one diplomat said, speaking in return for anonymity because he was not authorized to comment publicly.

Some analysts argue that, by slowing the growth of its stockpile, Tehran could delay the moment when it acquires sufficient 20 percent enriched uranium to trigger a response by Israel, which has signaled readiness to attack Iran’s nuclear sites.

The likely outcomes of the forthcoming sets of negotiations remain unclear.