By John Irish and Simon Lewis
PARIS/BERLIN (Reuters) - The United States and its European allies said on Thursday that it was now just a matter of weeks to salvage the 2015 Iran nuclear deal after a round of talks in which a French diplomatic source said there had been no progress on the core issues.
Indirect talks between Iran and the United States on reviving the nuclear deal resumed almost two months ago.
Western diplomats have previously indicated they were hoping to have a breakthrough over the next few weeks, but sharp differences remain with the toughest issues still unresolved. Iran has rejected any deadline imposed by Western powers.
Diplomats and analysts say the longer Iran remains outside the deal, the more nuclear expertise it will gain, shortening the time it might need to race to build a bomb if it chose to, thereby undermining the accord's original purpose. Tehran denies it has ever sought to develop nuclear arms.
"We are indeed at a decisive moment," U.S. Secretary of State Antony Blinken told a news conference after meeting British, French and German ministers in Berlin.
"There is real urgency and it's really now a matter of weeks, where we determine whether or not we can return to mutual compliance with the agreement."
Little remains of the 2015 deal, which lifted sanctions against Tehran in exchange for restrictions on its nuclear activities. In 2018, then-President Donald Trump pulled Washington out of the deal, reimposing U.S. sanctions, and Iran later breached many of the deal's nuclear restrictions and kept pushing well beyond them.
U.S. President Joe Biden, who succeeded Trump a year ago, said on Wednesday that it was not the time to give up because some progress was being made.
The current round of talks resumed on Dec. 27 after the adding of some new Iranian demands to a working text.
Western states have repeatedly said time was running out without setting a deadline for the end of talks. German Foreign Minister Annalena Baerbock again said the window of opportunity was closing.
"The negotiations have now entered a decisive phase. We need to make very, very urgent progress here, otherwise we will not be able to reach an agreement together that will bring sufficient added value to the central issue of non-proliferation," she said.
Highlighting the urgency, French Foreign Minister Jean-Yves Le Drian, said progress had been limited.
"There is partial, timid and slow progress, but negotiations cannot continue at such a slow pace while, in parallel, Iran's nuclear program advances so rapidly," he said in a statement. "We will quickly find ourselves in an untenable situation."
Iran refuses to directly meet U.S. officials, meaning that other parties - Britain, China, France, Germany and Russia - must shuttle between the two sides.
A French diplomatic source briefing reporters after the Berlin talks said such progress as had been made did not cover the most important topics "at the heart of the negotiation."
He would not set a deadline, but said the current trend was unsustainable.
"It seems necessary to us to change approach. I think that the month of February will be absolutely decisive. We are not going to continue like this in Vienna on the current trajectories in March, April, May etc."
Writing on Twitter, the lead U.S. negotiator in the talks, Rob Malley, said time was running out. "There is only a little time to reach and implement an understanding before developments in Iran’s nuclear program foreclose the possibility."
(Reporting by John Irish; Editing by Jan Harvey, William Maclean and Howard Goller)