Iran and Russia were hopeful about talks to rescue the landmark Iran nuclear deal a day after talks reopened in Vienna, while European negotiators emphasized the urgency of the talks, warning they were moving too slowly.
Negotiations to revive the 2015 accord began earlier this year but came to a halt in June when Iran's new ultraconservative government was elected. They began again in late November, with the most recent round beginning on Monday in Vienna.
The goal is to reintroduce the deal after Washington pulled out in 2018, as well as to curb Tehran's nuclear operations, which have accelerated in response to the US departure and reimposed sanctions.
"This is a critical negotiation... We are clearly approaching a point where Iran's escalation of its nuclear programme will have utterly demolished the JCPoA "In a statement, negotiators from the United Kingdom, France, and Germany referred 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 European countries, Iran, China and Russia are also taking part in the talks, and the United States is participating indirectly.
Iran insists all US sanctions must be lifted before steps are taken on the nuclear side, while Western negotiators say nuclear and sanctions steps must be balanced in the agreement.
Iranian Foreign Minister Hossein Amirabdollahian said a "good agreement for all parties" was possible in the near future if other parties showed "good faith", while Russian envoy Mikhail Ulyanov said a working group was making "indisputable progress" in the eighth round of talks.