The assessment came after Iran Foreign Ministry spokesman Nasser Kanaani was quoted by state broadcaster IRIB on Friday as saying his country had sent a response, through the EU, that had a “constructive approach aimed at finalising the negotiations”.
The US disagreed, with State Department spokesman Vedant Patel saying Washington was “studying it and will respond through the EU but, unfortunately, it is not constructive”.
The EU drafted a proposal to rescue the agreement, which was abandoned in 2018 by former US Donald Trump, and energy traders have been closely following the talks to see whether there will be a breakthrough.
White House National Security Council spokeswoman Adrienne Watson said some gaps had closed in recent weeks but others remained.
The IRIB report said Iran's response was sent to EU foreign policy chief Josep Borrell, who has been co-ordinating the negotiations. It gave no further details.
Stronger guarantees needed
Mr Borrell said on August 8 that the EU had made a final offer after 16 months of indirect talks between Iran and the US.
On Wednesday, Iran's Foreign Ministry said it needed stronger guarantees from Washington, with Mr Kanaani calling on the UN atomic watchdog to drop its “politically motivated probes” of the country's nuclear work.
Under the 2015 pact, Iran curbed its nuclear programme in exchange for relief from US, EU and UN sanctions.
Mr Trump reneged on the deal, arguing that it was too generous to Tehran.
He reimposed US sanctions on Iran, leading the country to resume previously banned nuclear activities and reviving US, European and Israeli fears that Iran could seek an atom bomb.
Iran denies any such ambition.