Iran has been engaged in negotiations to revive the accord with Britain, China, France, Germany and Russia directly and the United States indirectly since April 2021.

The EU diplomat, who coordinates talks between Iran and the US, is due to arrive in Tehran on Saturday night, state news agency IRNA reported. He is scheduled to meet Iran's chief nuclear negotiator Ali Bagheri on Sunday.

"Working on closing the remaining gaps in the #ViennaTalks on the #JCPOA," Mora tweeted ahead of his trip. "We must conclude this negotiation. Much is at stake."

Concerned parties have signalled for weeks that the negotiations are close to an agreement, but that "political decisions" are required from Tehran and Washington.

Western parties have been pushing for the talks to be concluded "urgently" given the accelerated pace of Iran's nuclear programme.

Following the United States' unilateral withdrawal from the deal in 2018 and the re-imposition of crippling economic sanctions, Iran began rolling back on most of its commitments under the accord from 2019.

The negotiations have been paused since March 11, after Russia demanded guarantees that Western sanctions imposed on it following its invasion of Ukraine would not damage its trade with Iran.

Days later, Russian Foreign Minister Sergei Lavrov said during a joint news conference with his visiting Iranian counterpart Hossein Amir-Abdollahian that Moscow had received the guarantees.

Consultations between Amir-Abdollahian and his counterparts had continued since delegations returned to their capitals, IRNA said.

- 'Difficult issues' -

The 2015 agreement gave Iran much-needed sanctions relief in exchange for curbs on its nuclear programme that would guarantee that Tehran could not develop a nuclear weapon -- something it has always denied wanting to do.

During the negotiations, Iran has repeatedly called for guarantees from the United States that there will be no repeat of its 2018 pullout.

That is one of two key demands that experts believe Iran is holding out for, with other being the removal of its Revolutionary Guards from a US terrorist blacklist.

The US said on Tuesday that it was now up to Iran to make hard decisions in order to restore the deal.

"The onus is on Tehran to make decisions that it might consider difficult," State Department spokesman Ned Price said.

"There are a number of difficult issues that we are still trying to work through."

Iran's top diplomat reiterated on Wednesday calls for the US to take steps to conclude the deal.

"Although Americans claim that they are interested in resolving issues and reaching an agreement, they must also show goodwill in practice," Amir-Abdollahian said.

Tehran would not give up its "red lines, which are not sacrificing the interests and rights of the people", he added.