Trump has ordered a 90-day review, saying that Iran was "not living up to the spirit" of the "terrible" deal
Iran and major powers were set to review adherence to their 2015 nuclear agreement on Tuesday, as uncertainty grows about the landmark accord's future under US President Donald Trump.
The regular quarterly meeting was expected to hear, as Washington confirmed last week, that Iran is sticking to its deal with the US, Russia, China, Britain, France and Germany.
The accord saw Tehran drastically curb its nuclear activities in return for the lifting of Western and UN sanctions.
However, Trump has ordered a 90-day review, saying last Thursday that Iran was "not living up to the spirit" of the "terrible" deal because of its actions in other areas.
This refers to Iran's support for Syrian President Bashar al-Assad, rebels in Yemen and militias in Iraq and in Lebanon as well as Tehran's ballistic missile programme.
US State Department spokesman Mark Toner said Monday the review would examine the nuclear accord "in the larger context of Iran's role in the region and in the world, and then adjust accordingly."
Trump's Secretary of State Rex Tillerson last Wednesday expressed misgivings about the nuclear deal itself, in particular time limits in key areas.
Iran cut the number of centrifuges that "enrich" uranium - making it suitable for power generation and at high purities for a bomb -- from about 19,000 to 5,000.
Together with other restrictions and ultra-tight UN inspections, Iran pledged to stay at this level for 10 years and not to enrich uranium above low purities for 15 years.
Its uranium stockpile will also stay below 300 kilograms - well short of what would be needed for an atomic bomb -- for 15 years.
Tillerson said that the accord "fails to achieve the objective of a non-nuclear Iran" and had been a way of "buying off" Tehran "for a short period of time".
Tehran not satisfied
Iran is not happy either, with critics of President Hassan Rouhani -- facing a tough battle for re-election next month -- charging that the nuclear deal has failed to provide all the promised economic benefits.
While nuclear-related sanctions were lifted, those related to human rights or missiles remained or have been expanded, frustrating Iran's efforts to boost trade.
Last week Foreign Minister Mohammad Javad Zarif responded to Trump's comments by saying that Washington was failing to live up not just to the spirit of the nuclear deal, but its wording too.
"So far, it has defied both," Zarif said on Twitter.
Tuesday's "Joint Commission" meeting from 0930 GMT among senior diplomats was to be held behind closed doors -- in the same plush Vienna hotel where the deal was hammered out -- with no press events planned.