It is the latest in a back and forth between the adversaries via mediators from the European Union, whose foreign policy chief Josep Borrell on Wednesday had hoped for a resolution within days to revive the 2015 nuclear accord.

The European Union put forward on August 8 what it called a final text. Iran proposed changes to it -- largely accepted by the Europeans -- to which the United States issued a response through the mediators.

Iranian foreign ministry spokesman Nasser Kanani said earlier Thursday that Tehran sent another reply in response to the US position, according to state news agency IRNA.

"We can confirm that we have received Iran's response through the EU," State Department spokesman Vedant Patel said late Thursday in Washington.

"We are studying it and will respond through the EU, but unfortunately it is not constructive."

The United States has declined to discuss details publicly but disputes include Iran's insistence on the UN nuclear watchdog closing a probe of three undeclared sites suspected in nuclear work before the 2015 nuclear deal.

President Joe Biden took office aiming to revive the accord, which was trashed in 2018 by his predecessor Donald Trump, who unleashed a volley of fresh sanctions on the cleric-run state.