President Hassan Rouhani on Sunday inaugurated a replacement factory at Natanz as well as a cascade of centrifuges for enriching uranium and two test cascades, in a ceremony broadcast by state television.
The accord guarantees relief from economic restrictions on condition that Tehran verifiably curbs most of its nuclear programme, in light of the purported nuclear proliferation risks posed by the Islamic Republic.
"We saw some signs of it, but certainly not enough", the official went on, stating that the original nuclear deal entitled the United States to sanction Iran on grounds that were not necessarily nuclear-related.
A senior State Department official told reporters after the conclusion of the talks this week, "The parties will reconvene next week in Vienna sometime mid-week".
A delegation from Washington flew to Vienna early this week, as Iran held formal meetings with signatories to the nuclear accord - Russia, China, the UK, France and Germany.
Representatives from major world powers and Iranian officials continued talks on the 2015 Joint Comprehensive Plan of Action (JCPOA) on Friday in Vienna, with the U.S. and Germany both expressing cautious optimism about the state of the ongoing negotiations.
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During a press appearance on Tuesday, Biden said he is updating his self-imposed deadline of May 1. The new deadline is two weeks ahead of Biden's previous deadline.
Then-president Donald Trump nixed the agreement in 2018 and started imposing sanctions as part of his maximum pressure campaign. European intermediaries relayed messages to the Americans from the talks.
The US official said the Vienna talks will resume next week with hopes of finding a solution to the divergent views. The U.S. side did not present a list of specific sanctions that could be lifted, and the Iranian side did not present a list of nuclear steps it would be willing to take to reverse JCPOA violations, the official added.
The United States says it is prepared to lift "sanctions that are inconsistent with the JCPOA".
Iran's lead delegate to the talks, Deputy Foreign Minister Abbas Araghchi, was quoted on Friday by Iranian state news site Press TV as saying Tehran will "only come back to full compliance with the agreement after the US lifts sanctions imposed, reimposed or relabeled following the USA withdrawal from the JCPOA". "It's just going to be much harder to for them to get what they say they want which is a return, mutual return, to compliance".
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. While it has declined to elaborate, that appears to exclude sanctions formally unrelated to nuclear issues covered by the deal.
After the explosion previous year, Iran said "sabotage is certain", adding there were strong suspicions that "internal agents" played a role.