"It could be argued that the Iranian nuclear program has actually accelerated in recent weeks and months. That's why a very firm posture at this time is utterly critical to make diplomacy work," said Dore Gold, a former Israeli ambassador to the United Nations.
Gold currently heads the Jerusalem Center for Public Affairs, a private think tank, but often acts as an informal adviser to Netanyahu.
Israel itself has not publicly defined its own red lines. Officials say that by doing so, they would essentially be telling the world when it is going to attack.
But Gold said potential red lines for the Americans would be an Iranian decision to enrich uranium beyond the current level of 20 percent, a clear signal they are on the road to weapons, or the accumulation of specific quantities of lower enriched materials.
Gold played down the differences with the U.S. "Basically we are in full agreement about how this is supposed to turn out. But there is a serious problem with respect to the strategy over how to influence Iran," he said.
Jahn reported from Vienna. Bradley Klapper in Washington and Ian Deitch in Jerusalem contributed reporting.
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