Iran and major world powers met Tuesday to take stock of their 2015 nuclear accord, the future of which is clouded by the imminent inauguration of Donald Trump as US president and the death of a moderate former Iranian president.

Trump, who will be inaugurated on January 20, has vowed to "dismantle" the "disastrous" accord, which saw Iran reduce its atomic activities in order to make any dash to develop nuclear weapons much harder.

The death of Akbar Hashemi Rafsanjani on Sunday aged 82 removed a widely respected backer in Iran of the deal at a time when frustration about the slow pace of reciprocal sanctions relief is growing.

Iranian news agency ISNA called his passing a "great loss for the moderates", describing the ayatollah and president from 1989 to 1997 as "the sheikh of moderation".

Tuesday's meeting in Vienna -- the city where the hard-fought deal was struck in July 2015 -- brought together senior diplomats from Iran and the United States, Russia, China, Britain, France and Germany.

It is the fourth such gathering to review progress implementing the pact since it came into force in January 2016 and was requested by Iran in December after US sanctions legislation was renewed for a decade.

US restrictions under the Iran Sanctions Act targeting mostly Tehran's oil and gas sectors remain suspended but the Islamic Republic still saw the move as a "violation" of the nuclear deal.

There is also disappointment in Iran that many of the hoped-for economic benefits from the deal have failed to materialise, putting President Hassan Rouhani under pressure ahead of elections in May.

Iran has been able to ramp up its vital oil exports but US sanctions related to non-nuclear issues remain in place, making foreign banks reluctant to handle large transactions with Tehran.

The UN watchdog has said Iran is meeting its side of the bargain, although it has twice inched above agreed ceiling on heavy water, a reactor coolant, and is near to an upper limit on uranium, diplomats say.

- All eyes on Trump -

But all bets will be off if Trump scraps what he has called "the worst deal ever negotiated".

Like many fellow Republicans, Trump is deeply suspicious of Iran and he looks set to be a close ally of Israel, widely thought to have nuclear weapons itself and whose government virulently opposed the Iran accord.

Since his stunning election win in November, Trump has stopped short of providing detailed plans on the subject, but his pick to head the CIA, Mike Pompeo, has made his opinions clear.

"I look forward to rolling back this disastrous deal with the world's largest state sponsor of terrorism," Pompeo tweeted the day before his nomination.

However, unilaterally ripping up the agreement might not be so easy, not least because the deal was endorsed by Russia, with whom Trump wants to improve relations, and the rest of the UN Security Council in a resolution.

"The election campaign is one thing, but a new stage when he enters office is another," Russia's envoy in Vienna to the UN, Vladimir Voronkov, told Russian news agency TASS.

"Then he (Trump) will get an opportunity to have another look at the document, at how it is implemented to decide his position on it in the future," Voronkov said.

Arms Control Association analyst Kelsey Davenport said that rather than unilaterally renouncing the deal, a more likely approach by Trump might be to "slowly chip away" at it.

This, Davenport said, could "create an escalatory dynamic that eventually provokes Iran into taking action that leads to the deal's collapse."

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