Officials have said the deal would become obsolete within weeks if Iran continued to step up its nuclear activities as it has been doing since 2019, a year after the United States left the landmark agreement and reimposed sanctions.

"Vienna talks to resume on Monday 27 December. The JCPOA Joint Commission will meet to discuss and define the way ahead," EU diplomat Enrique Mora wrote on Twitter, referring to the acronym of the deal's formal name.

"Important to pick up the pace on key outstanding issues and move forward, working closely with the US. Welcome to the 8th round."

Negotiations restarted in November after a five-month hiatus to try to restore the deal, which was to offer Iran sanctions relief in exchange for curbs on its nuclear programme.

Diplomats from parties to the deal -- China, Britain, France, Germany and Russia -- are in talks in Vienna with Iran and the US, with the two sides refusing direct contact.

US negotiator Rob Malley on Tuesday warned of a "period of escalating crisis" if diplomacy failed to restore the agreement.

Iran claims it only wants to develop a civilian nuclear capability, but Western powers say its stockpile of enriched uranium goes well beyond that and could be used to develop a nuclear weapon.