All sides have signalled progress in the talks being held in Vienna, but Iran has said that the West still needs to decide on some key issues.

"There is a critical urgency to conclude the negotiations this week," a foreign ministry spokeswoman said.

Parties to the 2015 deal saw it as the best way to stop the Islamic republic from building a nuclear bomb -- a goal Tehran has always denied.

Iran's chief negotiator Ali Bagheri has returned to Vienna after going home last Wednesday for consultations, during which talks continued at the level of experts in Vienna.

The negotiations to revive the deal, known formally as the Joint Comprehensive Plan of Action (JCPOA), involve Iran as well as France, Germany, Britain, Russia and China directly, and the United States indirectly.

The 2015 agreement gave Iran sanctions relief in exchange for curbs on its nuclear programme, but the US unilaterally withdrew from it in 2018 under then-president Donald Trump who reimposed heavy economic sanctions.

That prompted Iran to begin rolling back on its own commitments.

During the Vienna talks, Iran has repeatedly called for guarantees from the US administration of President Joe Biden that there will be no repeat of Trump's pullout.

State Department Spokesman Ned Price said Monday that the talks are at a "decisive consequential moment" and that the United States wants to see "additional clarity in the coming days" on Tehran's position.

Price noted that Iran's nuclear advances could soon render "meaningless" key benefits of the JCPOA.

"We are prepared to walk away if Iran displays intransigence to making progress," he said, noting that the US and allies have a Plan B readied if the talks fail.