Evidence began emerging last summer that the Iranians were diverting a significant portion of their medium-enriched uranium for use in a small research reactor, converting it into a form that cannot easily be used in a weapon.

One American official said the move amounted to trying to “put more time on the clock to solve this,” characterizing it as a step “you have to assume was highly calculated, because everything the Iranians do in a negotiation is highly calculated.” Israel’s departing defense minister, Ehud Barak, came to a similar conclusion when he said in October that his country could safely back away from threats of military action against Iran, at least until the late spring or summer of 2013.

But White House, State Department and Pentagon officials all cautioned against drawing firm conclusions about Iran’s ultimate intentions.