A first round of talks in Vienna aimed at reviving the Iran nuclear agreement made halting progress and will resume next week, with diplomats describing the end of four days of meetings on April 9 as constructive.

The EU-hosted talks center on overcoming an impasse between the United States and Iran to bring both parties into full compliance with the 2015 agreement, which lifted international sanctions on Tehran in exchange for limits on Iran's nuclear program.

President Joe Biden has expressed a willingness for the United States to reenter the accord first abandoned in 2018 under then-President Donald Trump, who imposed a raft of sanctions on Tehran under a "maximum pressure" campaign. Iran responded to the U.S. exit from the deal, known as the Joint Comprehensive Plan of Action (JCPOA), by gradually breaching many of the nuclear restrictions.

Although U.S. and Iranian diplomats did not hold face-to-face meetings, the other parties to the deal -- Britain, China, France, Germany, Russia - engaged in shuttle diplomacy to narrow differences.

Two working groups have been formed to hammer out a compromise, which if reached could still be weeks away. One expert group is focused on how to lift U.S. sanctions to bring Washington back into compliance with the accord. Another group is tasked with detailing a path for Iran to comply with restrictions on its nuclear program, including limits on uranium enrichment and centrifuges.

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An EU statement after the last session on April 9 said the so-called Joint Commission on the JCPOA had been 'briefed on the work of the two expert groups on sanctions lifting and nuclear implementation measures and participants noted the constructive and results oriented exchanges.'

It said 'the participants emphasized their resolve to further pursue the ongoing joint diplomatic effort' and that a coordinator under EU High Representative for Foreign Affairs Josep Borrell 'will continue his separate contacts with all JCPOA participants and the United States.'

The U.S. and Iranian sides have publicly clashed over the sequencing of possible U.S. sanctions relief and Iran reversing its breaches of the deal.

Iran demands the United States lift all sanctions and return to full compliance first, after which it says it will reciprocate.

"All Trump sanctions were anti-JCPOA & must be removed-w/o distinction between arbitrary designations," Iranian Foreign Minister Javad Zarif wrote on Twitter.

In addition to reimposing nuclear-related sanctions, the Trump administration slapped a web of sanctions on Tehran over a range of issues such as terrorism, human rights, and ballistic missiles. They include sanctions on Iran's Central Bank and the Islamic Revolutionary Guard Corps, a part of Iran's armed forces that the Trump administration labeled a terrorist organization.

Analysts say the bevy of sanctions were meant to deliberately complicate a future administration's efforts to rejoin the nuclear accord.

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Briefing reporters after talks wrapped up on April 9, a senior State Department official said the initial talks were encouraging but that the United States would not meet Iranian demands to lift all sanctions.

'If Iran sticks to the position that every sanction that has been imposed since 2017 has to be lifted or there will be no deal, then we are heading towards an impasse,' the senior U.S. official told reporters on a conference call.

The official said the Trump administration's sanction policy on Iran had a "purposeful and self-avowed intent to make it difficult for any future administration" to return to the nuclear deal.

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.

German Foreign Minister Heiko Maas told news network RND in comments published on April 10 that the talks were 'constructive' but cautioned they could drag on.

'All sides showed a willingness to work with the necessary sincerity towards the same goal -- the full implementation of the nuclear deal with Iran,' Maas said.

'It won't be easy. We are only at the beginning of intensive negotiations,' he said, adding that the talks concern highly complex issues and will require compromise.

Russia's ambassador to the UN in Vienna, Mikhail Ulyanov, said the parties 'took stock of the work done by experts over the last three days and noted with satisfaction the initial progress made.'

He later tweeted that representatives 'will reconvene next week in order to maintain the positive momentum.'

With reporting by AFP, AP, dpa, and Reuters

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