US President Donald Trump's UN envoy Nikki Haley told the world's atomic watchdog Wednesday that Washington has "concerns" about Iran's adherence to the 2015 nuclear deal.
Haley "praised the IAEA's technical expertise and its credibility, professionalism and seriousness in conducting its monitoring and verification work in Iran," a statement from her office said.
But at the talks in Vienna she and International Atomic Energy Agency chief Yukiya Amano "discussed US concerns about ensuring Iran strictly adheres to its obligations."
The statement said that "IAEA reports (about Iran's adherence to the accord) can only be as good as the access Iran grants to any facility the IAEA suspects of having a nuclear role."
The communique did not elaborate but this is likely a reference to the IAEA visiting military sites where the watchdog might suspect efforts to create an actual nuclear warhead. 
Iran denies wanting the bomb and the IAEA is not thought to have requested any such visits since the nuclear deal entered into force in January 2016.
Its regular reports focus on Iran's declared nuclear activities and say only that it is continuing to monitor Tehran's commitments not to conduct any weapons-related research.
The IAEA did not comment after Wednesday's meeting.
The 2015 deal between Iran and six major powers saw the Islamic republic curtail its uranium enrichment and plutonium capacities and submit to closer IAEA inspections.
This is aimed at making any covert dash to make a nuclear bomb extremely difficult.
Most UN and Western sanctions on Iran were lifted in return, but others related to non-nuclear issues have remained in place or been ratcheted up.
With Trump slamming the 2015 deal as "terrible", tensions have risen between the two long-time foes, with each accusing the other of not adhering to the "spirit" of the accord.
Trump is due in October to certify to Congress whether Iran is sticking to the deal. 
In July he told the Wall Street Journal he "would be surprised if they were in compliance".
The Washington Post on Tuesday quoted Haley as saying "no decision" has been made on the future of the deal, which also included Russia, China, France, Britain and Germany.
Trump's outspoken remarks about the accord, during last year's election campaign and since he entered the White House, have been sharply criticised by many nuclear experts. 
"The Trump administration needs a wake-up call on the costs of sabotaging the nuclear deal with Iran," Arms Control Association analyst Kelsey Davenport told AFP.
"Hopefully visiting the IAEA will allay concerns about monitoring the agreement and demonstrate to Haley that the deal put Iran's nuclear programme under a microscope and keeping it there is the best way to guard against any illicit nuclear activity," she said.