US Secretary of State Rex Tillerson has said Washington continues to have "significant issues" with the Iran nuclear deal, after his first talks with his Iranian counterpart.
But Mr Tillerson said the talks were "very matter of fact", and acknowledged Iran was complying with the deal.
Wednesday's talks at the UN, involving all seven parties that negotiated the deal, focused on its implementation.
They came a day after President Donald Trump denounced the deal.
He has until 15 October to decide whether to certify that Iran is complying with the deal. If he fails to do so, Congress could re-introduce sanctions dropped when the accord was implemented.
The New York Times quoted unnamed administration officials as saying Mr Trump will revisit the deal rather than scrapping it altogether.
Mr Tillerson said Mr Trump found the so-called sunset clause, which sees restrictions on Iran's nuclear enrichment programme lifted after 2025, unacceptable.
Iran reached the landmark nuclear deal with the P5+1 group of world powers - the US, UK, France, China and Russia plus Germany, in 2015.
Before the deal there were two uranium enrichment facilities in Iran - Natanz and Fordo - where uranium hexafluoride gas was fed into centrifuges to separate out the most fissile isotope, U-235.
Low-enriched uranium, which has a 3%-4% concentration of U-235, can be used to produce fuel for nuclear power plants, but it can also be enriched to the 90% needed to produce nuclear weapons.
In July 2015, Iran had almost 20,000 centrifuges. Under the Joint Comprehensive Plan of Action (JCPOA), it will be limited to installing no more than 5,060 of the oldest and least efficient centrifuges at Natanz for 10 years.
The centrifuges at Fordo now only produce radioisotopes for use in medicine, agriculture, industry and science.
Iran's uranium stockpile will also be reduced by 98% to 300kg (660lbs) for 15 years. It must also keep its level of enrichment at 3.67%.