The United States and its European allies said on Thursday that it was now just a matter of weeks to salvage the 2015 nuclear deal.
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.
"We are indeed at a decisive moment," US Secretary of State Antony Blinken told a news conference after meeting French, German and British 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."
The eighth round of talks, the first under Iran's new hard-line President Ebrahim Raisi, resumed on Dec. 27 after adding some new Iranian demands to a working text.
Western states have repeatedly said that time was running out without setting a deadline for the end of talks.
Germany's 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.