European negotiators stressed Tuesday that talks to save the landmark Iran nuclear deal are "urgent", the day after discussions between world powers resumed in Vienna.

Negotiations to restore the 2015 agreement began earlier this year but stopped in June as Iran elected a new ultra-conservative government. They resumed in late November with the latest round getting underway on Monday. 

The aim is to bring back the United States, which left the deal in 2018 and curtail Iran's nuclear activities, step up in response to the US withdrawal, and reimpose sanctions.

"This negotiation is urgent... We are clear that we are nearing the point where Iran's escalation of its nuclear program will have completely hollowed out the JCPoA," negotiators from Britain, France, and Germany said in a statement, referring to the deal's official name by its acronym.

"That means we have weeks, not months, to conclude a deal before the JCPoA's core non-proliferation benefits are lost."

Besides the so-called E3 countries and Iran, China and Russia are also taking part in the talks, while the United States is participating indirectly.

EU diplomat Enrique Mora, who is chairing the talks, said on Monday that all sides were showing "a clear will to work toward the successful end" but that "very difficult" negotiations lay ahead.