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The United States said Friday it offered on reviving the Iran nuclear accord but was waiting for Tehran to reciprocate as partner nations voiced optimism following talks in .

administration has opened indirect diplomacy with in hopes of returning to the 2015 agreement, which his predecessor Donald Trump trashed as he launched a "maximum pressure" campaign in hopes of bringing to its knees.

"The United States team put forward a very serious idea and demonstrated a seriousness of purpose on coming back into compliance if Iran comes back into compliance," a US official told reporters as talks broke for the weekend.

But the official said the United States was waiting for its efforts to be "reciprocated" by Iran.

"We saw some signs of it but certainly not enough. There's still question marks about whether Iran has the willingness to... take the pragmatic approach that the United States has taken to come back into compliance with its obligations under the deal," he said.

Biden argues that the 2015 nuclear deal negotiated under former president Barack Obama had been successful, with UN inspectors saying Iran was meeting its promises to scale back nuclear work dramatically.

Iran has demanded that the United States first lift all sanctions imposed by Trump, which include a sweeping unilateral ban on its oil exports, before it falls back in line with obligations it suspended.

The "US —- which caused this crisis —- should return to full compliance first," Iranian Foreign Minister Mohammad Javad Zarif wrote on Twitter, adding that "Iran will reciprocate following rapid verification."

The head of Iran's delegation to the talks Abbas Araghchi stressed the need for "political will and seriousness from other parties".

"Otherwise, there will be no reason to continue negotiations," he said, according to a statement from the Iranian foreign ministry.

Stumbling block over sanctions

The US official indicated that the major stumbling block in the initial talks was not the order of compliance but rather which sanctions were under discussion as Iran is demanding an end to all US restrictions.

The deal, formally known as the Joint Comprehensive Plan of Action, covers only nuclear sanctions and not US measures taken in response to human rights or other concerns, the official said.

"All sanctions that are inconsistent with the JCPOA and are inconsistent with the benefits that Iran expects from the JCPOA we are prepared to lift. That doesn't mean all of them because there are some that are legitimate sanctions," he said.

While Biden can lift sanctions, his diplomacy has already faced heated attacks from Trump's Republican Party, some of whose members have called in the past for attacking Iran.

Iran refused to meet directly with US negotiator Rob Malley during the talks led by the European Union, whose envoys shuttled between the two sides in different hotels.

Talks are set to resume Wednesday with Iran again meeting the other nations in the deal -- Britain, China, France, Germany and Russia as well as the European Union.

The EU diplomat leading the talks, Enrique Mora, said that the meetings had been "constructive and results oriented.”

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Moscow's ambassador to the UN in Vienna Mikhail Ulyanov also said that the participants "noted with satisfaction the initial progress made" and wanted to "maintain the positive momentum."

In another sign of easing tensions, Iran released a South Korean-flagged tanker that it had seized amid a dispute over billions of dollars in frozen oil funds, the foreign ministry in Seoul said.

Due to Trump's sanctions, South Korea had blocked $7 billion it owed Iran for past oil sales but it recently said it had resolved the dispute, subject to US approval.

US officials said they were not involved in the tanker's release and that the issue was not linked to the talks in Vienna.

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