The negotiations began last April between Britain, China, France, Germany, Iran and Russia, with the US taking part indirectly.

The EU diplomat chairing the talks Enrique Mora told reporters last week that delegations were down to negotiating the footnotes of the text, but progress stalled when Moscow demanded guarantees that Western sanctions over its invasion of Ukraine would not affect its trade with Iran.

However, Russian Foreign Minister Sergei Lavrov indicated on Tuesday that Russia had received "written guarantees" from the US.

That news was followed on Wednesday by the release of two British-Iranians after years of detention in Iran, taken as another sign the diplomatic climate was improving.

The increasingly positive signs have led some to hope that the revival of the 2015 deal may be just days away, with one diplomatic source saying the process was on "the right track".

However, the same source warned that "we have to be cautious".

With good reason -- negotiations over Iran's nuclear programme have been littered with missed deadlines over the years.

- Failed Russian 'gambit' -

The deal began to fall apart in 2018 when then US President Donald Trump withdrew from it and went on to re-impose swingeing economic sanctions on Iran.

That led Tehran to exceed the limits on its nuclear activity laid down in the deal.

Iran said on Wednesday that there were two sticking points that remained to be resolved at the Vienna talks, including an "economic guarantee" in case a future US administration repeats Trump's actions.

Another source close to the talks said the other issue revolved around the status of Iran's Revolutionary Guards, which Trump's administration classified as a terrorist organisation.

According to analyst Henry Rome from the Eurasia Group these problems are "unlikely to prove insurmountable".

"Both the US and Iran want a deal, and the latter probably used some diplomatic capital to persuade Russia to back off its confrontational stance," he adds, alluding to Iranian Foreign Minister Hossein Amir-Abdollahian's trip to Moscow to see Lavrov this week.

"It is now clear that Russia's tactical gambit to leverage the Iran nuclear deal to punch a hole in western sanctions regime over the crisis in Ukraine did not work," says Ali Vaez from the International Crisis Group.

The deal "is now too big to fail," Vaez says, adding that "too much time, energy and political capital has been expended" on progress thus far.

- Averting a 'major crisis' -

As ever with the talks, there is always the possibility of last-minute complications.

"There may yet be some theatrics, with Iran trying to leverage high oil prices to win several additional concessions," says Rome.

In addition, on Wednesday the UN nuclear watchdog the International Atomic Energy Agency issued a report giving fresh details of advances in Iran's production of uranium metal, which could complicate the implementation of a deal in the coming months.

Moreover as Daryl Kimball, executive director of the Arms Control Association told AFP, "with every passing hour, it's even more likely that the (Ukraine) conflict could complicate the talks".

Nevertheless, he says "it's clear that a restoration of the agreement is essential to avert another major global crisis".