But the official, who briefed reporters on condition of anonymity, stressed that the United States thought a deal remained possible even with Iran under a newly-elected hardline president, cleric Ebrahim Raisi.

"We still have serious differences that have not been bridged," the official said.

The official said the sides remained apart on central issues including what steps Tehran needed to take to get back into compliance with the original 2015 Joint Comprehensive Plan of Action (JCPOA) that lost effect after then-president Donald Trump withdrew three years ago.

Also still under discussion was what sanctions relief Washington would offer Iran, and the precise sequence of actions to be taken if an agreement is reached.

"Nothing is agreed until everything is agreed," the official said.

"But we wouldn't be going back for a seventh round if we didn't think a deal was possible."

US President Joe Biden pledged to return to the deal after succeeding Trump in January, and Washington believes that Iran wants that as well.

Negotiations resumed in April in Vienna with Britain, China, France, Germany and Russia, all also part of the original JCPOA.

EU negotiator Enrique Mora said on Sunday that those involved in the talks were "closer" to saving the Iran nuclear deal but that sticking points remained.

But the election of Raisi last week has raised questions over whether Tehran's stance will change.

Although he only takes office in August, he gave his support to the talks on Monday.

"Any negotiations that guarantee national interests will certainly be supported, but... we will not allow negotiations to be for negotiations' sake," he said.

Raisi's election "does not affect our determination to try to reach a deal," the US official said.

"The Iranians have been serious in the talks so far."

But the official stressed that though there was no deadline, the talks could not go on and on, especially as Iran continues to advance on its nuclear program.

"Certainly time is not a positive factor. This process won't be open indefinitely," the official said.