Western diplomats say they still have not had real nuclear negotiations with Iran, and unless there is swift progress that deal will soon be "an empty shell.”

Diplomats from the United Kingdom, France and Germany, forming the E3 group in the Vienna talks with Iran to revive the 2015 deal, expressed their concerns in a joint statement on Monday.

"As of this moment, we still have not been able to get down to real negotiations," the statement said. "Time is running out. Without swift progress, in light of Iran's fast-forwarding of its nuclear program, the JCPOA will very soon become an empty shell," they said, using the deal's full name, the Joint Comprehensive Plan of Action.

Iran returned to the talks at the end of November after suspending participation for five months. The new negotiating team representing hardline president Ebrahim Raisi refused to continue the talks on the basis of understandings reached in the previous rounds from April to June. They put forth new demands, which the United States and the E3 dismissed.

As time passes without an agreement, Iran continues to enrich uranium to 60-percent purity, getting closer to a level when it would need only weeks to have a large enough stockpile for a nuclear bomb.

With reporting by Reuters