GENEVA – The third round of nuclear talks between Iran and the six powers opened in Geneva Wednesday with the parties offering contradictory assessments of the chances for success.

Yet even as some of the negotiators sought to lower expectations, most seemed optimistic that by the end of the weekend, an agreement on curbing Iran’s nuclear program in exchange for sanctions relief would be reached.

A senior U.S. official insisted Wednesday that the proposal now on the table differs little from the one Iran rejected two weeks ago, even though lower-level talks with the Iranians have continued since then. The main sticking points last time were the Iranians’ rejection of the demand that they cease construction of their heavy-water reactor in Arak, and their insistence that the agreement explicitly recognize their right to enrich uranium.

U.S. Secretary of State John Kerry said Wednesday that the issue of whether Iran will ultimately be allowed to enrich uranium will not be decided in the interim deal under discussion in Geneva.

“Whatever a country decides or doesn’t decide to do, or is allowed to do under the rules, depends on a negotiation,” Kerry told reporters. “We are at the initial stage of determining whether or not there is a first step that could be taken, and that [the issue of Iranian enrichment] certainly will not be resolved in any first step, I can assure you.”

The talks in Geneva began with a luncheon attended by Iranian Foreign Minister Mohammad Javad Zarif and European Union foreign policy chief Catherine Ashton, who is heading the talks for the P5 + 1 countries – the United States, Russia, China, France, Britain and Germany.

A European Union spokesperson called the luncheon meeting “positive,” and the Iranian Fars News Agency quoted Zarif as calling the meeting “good.” But Zarif’s deputy, Abbas Araghchi, sounded less optimistic, telling Iranian reporters that the previous round of talks undermined trust between the sides, and that “the lost trust must be revived.” He also said the disagreement over Iran’s right to enrich uranium remains unresolved.

Representatives of some of the six powers also sounded more cautious about the chances of success.

A senior American warned that it would be “very hard” to clinch a breakthrough nuclear deal this week.

“I think we can (get a deal), whether we will, we will have to see because it is hard. It is very hard. ... If it was easy to do, it would have been done a long time ago,” the official said.

The French, for their part, were furious over remarks made Wednesday by Iran’s supreme leader, Ali Khamenei, in which he termed Israel a “rabid dog” and accused the French of “bending the knee to the Zionist regime” by taking a tough stance in the last round of talks. A spokeswoman for French President Francois Hollande denounced the remarks, saying they “complicated” the Geneva talks.

Yet over in Geneva, neither the senior American official nor Ashton’s spokesman followed Hollande’s lead, though the American admitted that Khamenei’s remarks caused some “discomfort.”

British FM: Gap is ‘narrow’

By contrast, Russian President Vladimir Putin said after a meeting with Prime Minister Benjamin Netanyahu in Moscow Wednesday that he is optimistic about the talks in Geneva, as did British Foreign Minister William Hague.

Hague, speaking in Istanbul at a joint press conference with Turkish Foreign Minister Ahmet Davutoglu, called the nuclear talks a “historic opportunity,” adding, “A deal is on the table that would be in the interest of all nations, including countries across the Middle East. The differences that remain between the parties are narrow and I believe they can be bridged through political will and commitment.”

Publicly, at least, there was little discussion Wednesday of the main issues on the table – the Arak reactor and what will be done with Iran’s 200 kilograms of uranium enriched to 20 percent. The substantive talks on these issues are expected to begin only Thursday.

But the American official did shed some light on the other side of the equation: He said the sanctions relief would be “balanced, targeted, limited and reversible,” with the vast majority of sanctions remaining intact, and also promised that Washington would “vigorously” implement them.

The U.S. official also said he “understands [Prime Minister] Netanyahu has to call things how he sees them. We share a joint objective, preventing Iranian nuclear weapons.”

Netanyahu, in remarks after the meeting with Putin, said, “We believe it is possible to reach a better agreement, but it requires us to be consistent and persistent.”

The Ashton-Zarif luncheon was followed by a plenary meeting of the Iranian delegation with the P5 +1 negotiating teams. That meeting lasted only 15 minutes, indicating that the parties are agreed on matters of protocol.

The talks are expected to continue until Friday, but they may be extended if deemed necessary. Should the talks yield an agreement ready to be signed, the foreign ministers of the six countries will go to Geneva.