Talks to salvage the 2015 Iran nuclear deal in Vienna will resume on Monday, two parties to the talks said.

"Usually it isn't popular to engage in serious business b/w the Catholic Christmas and the New Year," Mikhail Ulyanov, Russia's top envoy to the talks, tweeted on Thursday.

"In this particular case this is an indication that all negotiators don't want to waist time and aim at speediest restoration of #JCPOA," he added, using the acronym for the deal, the Joint Comprehensive Plan of Action.

The European Union's foreign service said the meeting would be attended by representatives of China, France, Germany, Russia, Britain and Iran.

"Participants will continue the discussions on the prospect of a possible return of the United States to the JCPOA and how to ensure the full and effective implementation of the agreement by all sides," the External Action Service's statement said.

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 program.

Diplomats from the five countries to the deal are in talks in Vienna with Iran and the U.S., with the two sides refusing direct contact.

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

(With input from Reuters, AFP)