"For now, certainly there's no indication, positive indication that Iran is prepared to come back... and to try to close down the remaining issues," a senior US official said.

Western nations are trying to build momentum at the UN General Assembly in New York this week to kickstart the accord.

Then-president Donald Trump pulled America out of the deal in 2018, reinstating sanctions on Iran that Washington had lifted as part of the agreement.

Since then, Tehran has also retreated from many of its commitments.

Trump's successor Joe Biden has indicated he wants to return to the deal, but his administration has expressed impatience at the stalled talks.

"We don't have direct interaction with the Iranians so it's hard for us to assess levels of optimism and pessimism," said the official in a briefing to journalists.

He said "nothing has happened" to make the United States optimistic.

"We've not heard anything through indirectly about a date, or about Iran's intentions to continue the work that was begun in Vienna and try to close those gaps.

"The window opportunity is open and won't be open forever," he added.

The Joint Comprehensive Plan of Action is the agreement the United States, China, Russia, Germany, France and Britain reached with Iran on Tehran's nuclear program in 2015.

France Foreign Minister Jean-Yves Le Drian told reporters earlier this week on the sidelines of the annual UN General Assembly high-level meeting that a resumption of talks had not occurred because of Iran, which elected a new president in June.