President Hassan Rouhani officially inaugurated the cascades of 164 IR-6 centrifuges and 30 IR-5 devices at Iran's Natanz uranium enrichment plant in a ceremony broadcast by state television. Tehran says its nuclear programme is for peaceful purposes only.
Rouhani's actions may have been Iran's latest in a series of breaches of the 2015 nuclear deal Iran reached with world powers.
Iran is seeking to have US sanctions lifted, while the Biden administration is looking for the country to return to the terms of the 2015 Nuclear Deal. The talks began Tuesday with Iran meeting Britain, France, Germany, China and Russian Federation, the five other remaining signatories of the 2015 deal known as the Joint Comprehensive Plan of Action, and those five world powers meeting separately with the USA delegation nearby.
While Biden can lift sanctions, his diplomacy has already faced heated attacks from Trump's Republican Party, some of whose members have called in the past for attacking Iran.
Will the JCPOA finally emerge from oblivion under the new U.S. administration, nearly three years after former President Donald Trump universally withdrew from the Iran nuclear accord? Iran retaliated a year later by exceeding the JCPOA's nuclear activity limits.
What are European officials saying about the talks?This week, the State Department said the USA would be open to negotiations with Iran.
"It's really just preliminary steps", the official said.
The US' comments closely followed a tweet by Iranian Foreign Minister Javad Zarif, who said Iran would only comply with the deal again if all Trump-era sanctions were lifted.
"All sanctions that are inconsistent with the JCPOA and are inconsistent with the benefits that Iran expects from the JCPOA, we are prepared to lift", the official said, adding that a question remained about whether Iran is serious about reciprocating the USA offer.
The Vienna talks are focused not only on lifting crippling economic sanctions Trump reimposed, but also on bringing Iran back into compliance after it responded by suspending several of its own commitments.
"That doesn't mean all of them because there are some that are legitimate sanctions", he said.
US Secretary of Defense Lloyd Austin arrived in Israel on Sunday on the first visit by a senior representative of the Biden administration, whose stance on Iran has anxious Prime Minister Benjamin Netanyahu's government.
"Under the  deal, the USA retains the right to impose sanctions for non-nuclear reasons whether it is terrorism or human rights violations or interference with our elections et cetera", the official said.
The official noted that "it would be infinitely easier if we had direct conversations with Iran", but added that "if they don't want to meet with us, too bad".