Following the closed meetings of the signatories to the deal, known as the Joint Comprehensive Plan of Action, Russia's delegate, Mikhail Ulyanov, tweeted that the initial talks were "successful".
Iran's Deputy Foreign Minister Abbas Araqchi said after the talks that they were constructive and heading in the right direction. "They have left the JCPOA and they have imposed sanctions".
Rabiei added that Iranian representatives would address American counterparts only after the elimination of sanctions on the regime. "If US lifts all sanctions, Iran will then cease all remedial measures".
In a statement Sunday, however, the deputy Iranian foreign minister said there will be no "direct or indirect talks" with the US on the country's nuclear program throughout the week.
Also due in the Austrian capital is a US delegation headed by the administration's special envoy for Iran, Rob Malley.
Officials from Britain, France and Germany will act as intermediaries between Iran and the United States, shuttling between both delegations.
Then-President Donald Trump pulled the USA out of the accord in 2018, opting for what he called a maximum-pressure campaign of stepped-up US sanctions. In response, Iran has been producing nuclear materials beyond the limits set by the accord.
"Constructive Joint Commission meeting".
They are to start work immediately, and report their conclusions to the main negotiators. "The most important thing after today's meeting of the Joint Commission is that practical work towards achieving this goal has started".
Enrique Mora, the European coordinator for the talks, also called the meeting "constructive", vowing to "intensify separate contacts" with all relevant parties, including the United States, to get the deal back on track.
White House spokesperson Jen Psaki suggested that the negotiations will be a hard and long process.
Malley's remarks came as parties to the 2015 deal - including the USA and Iran, as well as British, French, and German representatives - gathered in the Austrian capital for a multi-day round of talks, with the latter delegations facilitating indirect talks between the Americans and Iranians.
Price said the talks will be "structured around working groups" that the European Union will form with Iran and the other parties to the deal.
He added that Washington is open to face-to-face talks with Tehran.
"What is promising about the Vienna talks is there seems to be an understanding now by both the Biden administration and the Iranians that neither side is going to go first". "Iran is fully ready to reverse its activities and return to complete implementation of the deal immediately after it is verified sanctions are lifted". Still, he defended efforts to reimplement the JCPOA in an interview with NPR on Tuesday.
Washington, for example, could unfreeze Iranian funds held in foreign banks and facilitate humanitarian trade, and Tehran could stop enriching uranium beyond the levels agreed to in the 2015 accord, said Davenport. We've seen the result of the maximum pressure campaign. "It has failed", Malley said.
On Tuesday, China appeared to back the Iranian position that U.S. sanctions must be lifted to revive the accord.
"The root cause of the Iranian nuclear crisis is the USA unilateral withdrawal from the #JCPOA which undermined the consensus of all parties", Chinese foreign ministry official Zhao Lijian wrote on Twitter.
"Lifting U.S. sanctions is the first and the most necessary action for reviving the deal", Araghchi was quoted as saying.
Tehran and Beijing had signed a 25-year "strategic cooperation" agreement late last month.
The Biden administration has also said it wants to build a "longer and stronger agreement" that would deal with other issues, including Iran's long-term nuclear programme, its development of ballistic missiles, and its support for proxy forces across the Middle East.
But the White House is facing pressure from Republicans and some Democrats against reviving the nuclear agreement.
"The Biden administration's first priority should be freeing American hostages, not offering sanctions relief to the ayatollahs".
In late February, Iran began restricting worldwide inspections of its nuclear facilities, but under a last-minute deal worked out during a trip to Tehran by Rafael Grossi, the head of the Vienna-based United Nations atomic watchdog, some access was preserved.