The Obama administration brushed off harsh criticism of Israel from Iran’s supreme leader Wednesday and insisted a proposed deal to curb Iran’s disputed nuclear program is not the trap Israel fears.

“We have the best chance we’ve had in a decade, we believe, to halt progress and roll back Iran’s program,” Secretary of State John F. Kerry said in Washington as U.S. and other negotiators began a new round of talks meant to clinch an interim agreement with Iran.

“We will not allow this agreement, should it be reached — and I say should it be reached — to buy time” for Iran or fall short of “our core fundamental concerns,” Kerry said.

He spoke hours after Iran’s supreme leader, Ayatollah Ali Khamenei, referred to Israel as the warmongering “rabid dog” of the Middle East during a speech to military commanders in Tehran.

Israeli Prime Minister Benjamin Netanyahu has waged a public lobbying effort against the draft deal, calling it a sham that rewards Iran without preventing it from producing a nuclear weapon.

Unable to persuade Washington to delay the latest round of talks, Netanyahu went to Russia on Wednesday seeking stronger limits on Iran.

Talks in Geneva convened with a senior U.S. negotiator telling reporters it will be difficult but not impossible to reach a deal.

“Whether we will, we will have to see, because it is hard,” the official said. “If it was easy to do, it would have been done a long time ago.”

Western governments suspect Iran has pursued a covert goal of enriching enough uranium to produce weapons on demand. Iran denies any weapons ambitions and claims it must continue to enrich uranium for peaceful energy uses.

A deal to satisfy international doubts about Iran’s program will turn largely on whether it can limit Iranian uranium enrichment to levels far below those used to make weapons. Negotiators are also seeking new controls on an Iranian plutonium reactor.

In return, world powers would first loosen and eventually lift some economic sanctions imposed over the decade Iran has defied international demands to roll back its program and provide more information.

Although Iran has long insisted on its right to enrich uranium on its own soil, the proposed deal is expected to fudge that point while stressing Iran’s right to peaceful nuclear energy.

“Whatever a country decides or doesn’t decide to do or is allowed to do and permitted under the rules depends on a negotiation,” Kerry said at a news conference with Australian officials.

“I’m not going to predetermine its outcome except to say to you that no right is recognized or granted within anything that I’ve seen in the early discussions,” he said.

The United States and other world powers had hoped to seal a deal earlier this month. Those talks broke up after disagreement first among the powers talking to Iran and then with Iran accusing France of torpedoing the deal.

Khamenei criticized France on Wednesday and vowed that Iran will not give up what he called its nuclear rights. He said he had set "red lines” for his negotiators and blamed Israel for stoking animosity around the talks.

“The real threats to the world are evil powers including the Zionist regime and its supporters,” Khamenei said.

“The Zionist regime is doomed to destruction, because this despicable regime was formed by power and imposed on the world, and nothing which is imposed will last,” he said.

The strong anti-Israel language may have been chiefly intended to reassure hard-liners at home, but it contrasts with the friendlier face presented by President Hassan Rouhani and Foreign Minister Mohammad Javad Zarif as they sought direct negotiations and a rollback of stringent international economic sanctions.

Jason Rezaian in San Francisco contributed to this report.