A European Commission handout picture shows (L-R) Seyyed Abbas Araghchi, Iranian Deputy Minister for Foreign Affairs in charge of Legal and International Affairs, Mohammed Javad Zarif, EU High Representative for Foreign Affairs Catherine Ashton, Helga Maria Schmid, Deputy Secretary General of the European External Action Service (EEAS), and James Morrison, Head of cabinet of Catherine Ashton, before the start of three days of closed-door nuclear talks at the United Nations offices in Geneva, Switzerland, on Tuesday. (ALAIN GROSCLAUDE/ EUROPEAN COMMISSION / HANDOUT/EPA)

Optimism for a nuclear deal with Iran seemed to wane Thursday, as diplomats from the Islamic republic and six major world powers struggled to find common ground on how to scale back Iran’s atomic energy program.

Diplomats sought to build on momentum from negotiations 10 days ago, when officials said they had come close to a preliminary agreement that would have halted the expansion of Iran’s nuclear facilities in exchange for modest relief from economic sanctions.

But the negotiators’ earlier confidence appeared to have faded, as diplomats traded barbs in television interviews and warned of significant gaps between the sides. Iran’s second-ranking official present at the talks said his delegation had “lost our trust” in its negotiating partners, because of changes in the terms of the proposed agreement offered by the so-called P5-plus-1 powers (the United States, Britain, China, France, Germany and Russia).

“We cannot enter serious talks until the trust is restored,” Iranian Deputy Foreign Minister Abbas Araghchi told reporters during a pause between sessions. “That doesn’t mean that we will stop negotiations.”

France’s chief diplomat, Foreign Minister Laurent Fabius, also sounded a pessimistic note, telling France 2 television that any deal could “only be possible based on firmness” and hinting that Tehran had not yet met the test.

“For now, the Iranians have not been able to accept the position of the six,” Fabius said. “I hope they will accept it.”

Despite the verbal jousting, a European Union spokesman described Thursday’s meetings as “substantial and detailed.” The talks included a private session between E.U. foreign policy chief Catherine Ashton and Iranian Foreign Minister Mohammad Javad Zarif. The Iranian side met briefly with U.S. diplomats in a bilateral session Wednesday night.

The tone of the meeting was soured, in part, by public comments Wednesday by Iran’s supreme leader, Ayatollah Ali Khamenei, who referred to Western countries as “evil powers” and called Israel the “rabid dog” of the Middle East.

The Obama administration brushed off the remarks and insisted that a proposed deal to curb Iran’s disputed nuclear program is not the trap that 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. He said the United States would not allow an agreement, should one be reached, to “buy time” for Iran to make progress toward a weapons program or to fall short of “our core fundamental concerns.”

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 enable it to produce nuclear weapons if it decided to do so. Iran denies any weapons ambitions and claims it must 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 heavy-water reactor that could potentially yield plutonium.

In return, world powers would first loosen and eventually lift some economic sanctions imposed over the decade that 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 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.”

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 Zarif, the foreign minister, as they sought direct negotiations and a rollback of stringent international economic sanctions.

Gearan reported from Washington. Jason Rezaian in San Francisco contributed to this report.