VIENNA. Talks on restoring a deal to curb Iran's nuclear programme and ease sanctions are near conclusion, a Russian envoy said on Tuesday, and sources close to the negotiations said a prisoner swap between Iran and the United States is expected soon.

"Apparently the negotiations on restoration of #JCPOA are about to cross the finish line," Mikhail Ulyanov said on Twitter, using the 2015 agreement's full name, the Joint Comprehensive Plan of Action.

Reuters reported last week that a US-Iranian deal was taking shape in Vienna after months of talks between Tehran and major powers to revive the nuclear deal pact, abandoned in 2018 by then-US President Donald Trump, who also reimposed extensive sanctions on Iran.

A draft text of the agreement alluded only vaguely to other issues, diplomats said, adding that what was meant by that was unfreezing billions of dollars in Iranian funds in South Korean banks, and the release of Western prisoners held in Iran.

On Saturday, Iranian Foreign Minister Hossein Amirabdollahian said the Islamic Republic was ready for an immediate prisoner exchange with the United States.

"Iran has always and repeatedly expressed its readiness to exchange prisoners. Months ago we were ready to do it but the Americans ruined the deal," a senior Iranian official in Tehran told Reuters, without elaborating.

"Now I believe some of them will be released, maybe five or six of them. But those talks about prisoners are not linked to the nuclear agreement, rather associated with it. This is a humanitarian measure by Iran."

US negotiator Robert Malley has suggested that securing the nuclear pact is unlikely unless Tehran frees four US citizens, including Iranian-American father and son Baquer and Siamak Namazi, that Washington says Tehran is holding hostage.

"Six years ago the Iranian government arrested Baquer Namazi and they still refuse to let him leave the country," Malley tweeted on Tuesday. "The Iranian government can and must release the Namazis, Emad Shargi, Morad Tahbaz, and other unjustly held US and foreign nationals."

Dozens of dual nationals, foreigners jailed in Iran

Iran, which does not recognise dual nationality, denies taking prisoners to gain diplomatic leverage. However, in recent years, the elite Revolutionary Guards have arrested dozens of dual nationals and foreigners, mostly on espionage and security-related charges.

Britain has been seeking the release of British-Iranians Anousheh Ashouri, jailed on espionage charges, and Nazanin Zaghari-Ratcliffe, a project manager with the Thomson Reuters Foundation who was convicted of plotting to overthrow the clerical establishment.

Tehran has sought the release of over a dozen Iranians in the United States, including seven Iranian-American dual nationals, two Iranians with permanent US residency and four Iranian citizens with no legal status in the United States.

Most were jailed for violating US sanctions against Iran.

In the latest comments on the final phase of 10 months of nuclear negotiations, the talks' coordinator, Enrique Mora, tweeted that "key issues need to be fixed" but the end was near.

Several Iranian officials, who spoke to Reuters on condition of anonymity, said some minor technical issues were being discussed in Vienna and that a deal was expected before the end of the week, though adding that "nothing is agreed until everything is agreed".

Separately, hardline Iranian President Ebrahim Raisi told gas exporter countries on Tuesday to avoid any "cruel" sanctions imposed by the United States on Tehran.

"The members of this forum should not recognise those sanctions...(because) in today's world we see that the sanctions are not going to be effective," Raisi told a gas exporters conference in Doha.

The 2015 deal between Iran and world powers limited Tehran's enrichment of uranium to make it harder for it to develop material for nuclear weapons, if it chose to, in return for a lifting of international sanctions against Tehran.

Since 2019, following the US withdrawal from the deal, Tehran has gone well beyond its limits, rebuilding stockpiles of enriched uranium, refining it to higher fissile purity and installing advanced centrifuges to speed up output.