The European Union on Tuesday voiced concern at Iran's announcement that it would resume uranium enrichment at an underground plant, in a fresh step away from the 2015 nuclear deal.
"We are concerned by President Rouhani's announcement today to further reduce Iran's commitments under the JCPOA," EU spokeswoman Maja Kocijancic told reporters.
Iranian President Hassan Rouhani said Tuesday that Iran would resume uranium enrichment at an underground plant south of Tehran in its latest step back from a troubled 2015 agreement with major powers.
The suspension of all enrichment at the Fordow plant in the mountains near the Shiite holy city of Qom was one of the restrictions on its nuclear activities that Iran accepted in return for the lifting of international sanctions.
But Washington's abandonment of the deal in May last year followed by its reimposition of crippling sanctions prompted Iran to begin a phased suspension of its own commitments in May this year.
Rouhani recalled that under the terms of the agreement Iran had retained more than 1,000 centrifuges at the plant which had been running empty since it went into effect.
"Starting from tomorrow (Wednesday), we will begin injecting (uranium hexafluoride) gas at Fordo," Rouhani said in a speech broadcast by state television.
His announcement came a day after tensions flared anew on the 40th anniversary of the US embassy siege and hostage crisis, with thousands in Tehran taking to the streets and Washington imposing fresh sanctions.
Iran said the resumption of enrichment at Fordow would be carried out transparently and witnessed by inspectors from the UN nuclear watchdog, the International Atomic Energy Agency.
But Russia, which has close ties with Iran, expressed concern about the latest move.
"We are monitoring the development of the situation with concern," President Vladimir Putin's spokesman Dmitry Peskov told reporters.
"We support the preservation of this deal."
At the same time, Peskov said Moscow understood Tehran's concerns over the "unprecedented and illegal sanctions" imposed by Washington.
The move is the fourth announced by Iran since it began responding to Washington's abandonment of its commitments.