Vienna__ A senior US official took a tougher line on prospects for reaching a final deal as negotiators from Iran and six world powers arrived here to begin the first drafting round towards a comprehensive Iran nuclear accord.

“Everyone comes to the table wanting a diplomatic solution,” the senior US administration official, speaking not for attribution, told journalists in Vienna Tuesday. “But having the intention does not mean it will happen.”

“Frankly, this is very, very difficult. Though we are drafting… it does not mean agreement is imminent,” the US official cautioned. “There are a range of complicated issues to address. We do not know if Iran will accept” taking the steps necessary.

I am not optimistic or pessimistic, but realistic, the US official said, in answer to a question about what seemed a notably less upbeat forecast about prospects for reaching a compromise than in recent earlier rounds focused more on agenda-setting. She was also reacting to what she said was speculation in the media about provisional agreement reached on aspects of a final deal, such as a solution to the Arak reactor, and a growing sense of optimism in media reports that a deal would be reached.