Senator John Kerry, chairman of the Senate Foreign Relations Committee, warned that the international community already has sent a message of "ambivalence if not impotence" by allowing Iran to reach a point where it is enriching uranium on an industrial scale.

"While Iran was just talking with the IAEA and the Europeans, it definitely sidestepped every red line laid down by the international community. While Iran was just talking to the world, it was moving to the threshold of becoming a nuclear state," he said.

Kerry opened the week-long hearings on US policy options in Iran just days after President Barack Obama promised "principled and sustained" engagement with Iran, a US bete noir for three decades.

Adding to the sense of urgency was a warning Sunday by the chairman of the US Joint Chiefs of Staff that Iran now has enough fissile material to build a nuclear bomb.

Kerry said the United States must set a timetable for "substantive progress" in any talks with Iran and back it with a credible threat of tougher international sanctions and further restrictions on trade and finance.

"Iran needs to understand these will not be drawn out negotiations," he said. "That's a scenario that gives Tehran a green light to make more progress on enrichment and other nuclear projects, some of them still being carried out in the dark."

Experts testifying before Kerry's committee, however, urged that the new administration take a more positive approach with the Iranians and avoid publicly setting red lines on Iranian behavior that could not be enforced.

Richard Haas, president of the Council on Foreign Affairs, expressed doubt that Iran would simply give up enrichment and suggested that Washington accept a "right to enrich."

"I believe then the negotiations would need to focus on whether Iran is allowed to have some enrichment capability. Or put another way, how the right to enrichment is defined -- what is the scale, what is the degree of transparency, what is the degree of IAEA access," Haas said.

"Sanctions would be directly linked to what it is Iran agreed to in terms of the scale of the program ... and transparency," he said.

But Senator James Risch, a Republican from Idaho, said it was "incredibly naive" to think Israel will stay on the sidelines if Iran closes in on acquiring nuclear weapons.

"This discussion needs to include what will happen when Israel does what I think it will inevitably do to keep Iranians from completing a nuclear weapon," Risch said, pointing to Israel's history of pre-emptively striking nuclear reactors in Iraq and more recently Syria.

Frank Wisner, a former ambassador, agreed that "any Iranian who doesn't take very seriously the Israeli threat to an Iranian nuclear capability is misjudging this nation's most vital interests."

But he said an Israeli military response to Iranian nuclear developments is "going to put us all in a really, really very difficult situation."

Iran's acquisition of a nuclear weapon likely would be shrouded in ambiguity, which Israel might decide not take chances with, he said. "But it would put us in terrific harm's way."

Other experts concurred that military action offered no solution to Iran's nuclear challenge, with some warning of dire repercussions for the United States and the region.

And they said there was still time for diplomacy despite the warning by Admiral Mike Mullen, the chairman of the Joint Chiefs of Staff, that Iran has stockpiled enough fissile material to make a nuclear weapon.

Mark Fitzpatrick, senior fellow as the London-based International Institute for Strategic Studies, told the committee the Iranian stockpile would still have to be further enriched to bomb grade, which would take several weeks.

"And the basic truth bears repeating, that having a stockpile of enriched uranium is not the same as having a bomb," he said.

It would take six months or more to fashion the highly enriched uranium into the pit for a nuclear weapons, he estimated.

Building only one weapon would be "a huge risk," he said, suggesting Iran may hold off on taking that step until it has sufficient material for more weapons.

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