Three EU countries on Tuesday launched a process charging Iran with failing to observe the terms of the 2015 deal curtailing its nuclear programme, a move that sparked anger and threats from Tehran at a time of intensifying tensions.
Russia also condemned the European move, warning it risked causing a "new escalation".
Britain, France and Germany insisted they remained committed to the agreement, which has already been severely tested by the US exit from the accord in 2018.
But even as their statement was released, British Prime Minister Boris Johnson indicated he might prefer a comprehensive accord negotiated with US President Donald Trump instead of the 2015 deal.
The decision to begin the so-called dispute mechanism process comes as tensions soar between the West and Iran following the killing of top commander Qasem Soleimani in a US strike, and the admission by Tehran days later that it had accidentally shot down a Ukrainian airliner.
The foreign ministers of the three European nations said Iran had been progressively scaling back its commitments under the deal since May last year.
"We have therefore been left with no choice, given Iran's actions," to begin the dispute process, their statement said.
In Moscow, the foreign ministry said it saw "no reason for such a move."
"We do not rule out that the thoughtless actions of the Europeans could lead to a new escalation around the Iranian nuclear accord," it said in a statement.
The 2015 nuclear deal signed in Vienna - known as The Joint Comprehensive Plan of Action (JCPoA) - has a provision that allows a party to claim significant non-compliance by another party before a joint commission.
If the issue is not resolved by the commission, it then goes to an advisory board and eventually to the UN Security Council, which could reimpose sanctions.