The United States on Friday confirmed it would take part in a meeting in Vienna next week on the Iran nuclear deal and offered to sit down directly with Tehran.

"These remain early days, and we don't anticipate an immediate breakthrough as there will be difficult discussions ahead. But we believe this is a healthy step forward," State Department spokesman Ned Price said. "We do not anticipate presently that there will be direct talks between the United States and Iran through this process, though the United States remains open to them," he said.

The European Union announced Friday an in-person meeting in Vienna of all parties to the 2015 nuclear deal, formally known as the Joint Comprehensive Plan of Action, from which former president Donald Trump withdrew.

The Europeans said it would have "separate" contacts in Vienna with the United States and Iran quickly rejected a direct meeting with its arch-enemy as it presses President Joe Biden first to lift sanctions.

Iranian Foreign Minister Mohammad Javad Zarif said the meeting would take place Tuesday and insisted the aim was to "rapidly finalise sanction-lifting and nuclear measures for choreographed removal of all sanctions, followed by Iran ceasing remedial measures."

"No Iran-US meeting. Unnecessary," he wrote on Twitter.

New US President Joe Biden has promised to rejoin the agreement on condition Iran first returns to respecting commitments it abandoned in retaliation for Trump pulling out. But Tehran says Washington has to lift international sanctions that were reimposed by Trump before it will make any moves to get back in line, and is refusing to hold direct negotiations with the US.

"The coordinator will also intensify separate contacts in Vienna with all JCPOA participants and the United States," it said, referring to the deal by its initials.

Price said that the "primary issues" for discussion in Vienna will be "the nuclear steps that Iran would need to take in order to return to compliance with the terms of the JCPOA, and the sanctions relief steps that the United States would need to take in order to return to compliance as well."

The Vienna talks will also include the governments of Britain, China, France, Germany and Russia which all remain parties and supporters of the nuclear deal negotiated under former US president Barack Obama.

'Substantial work ahead' -

Senior EU diplomat Enrique Mora, who chaired the talks, described Friday's virtual meeting as "positive", but warned there was much left to do to revive the deal. "Substantial work ahead for a key opportunity to bring JCPOA back to life," he tweeted.

The painstakingly negotiated deal saw Iran granted relief from international sanctions in exchange for accepting limits on its nuclear programme aimed at easing fears it could acquire an atomic weapon.

German Foreign Minister Heiko Maas said it was "good that all the relevant actors would meet in Vienna next week".

"We have no time to waste. A treaty that is fully respected once again would be a plus for security throughout the region," he said.

Russian diplomat Mikhail Ulyanov said "the impression is that we are on the right track but the way ahead will not be easy and will require intensive efforts. The stakeholders seem to be ready for that".

"Possible US return to JCPOA apparently will require Washington's full compliance with the nuclear deal," he wrote on Twitter.