Iran's parliamentary speaker on Sunday warned of unspecified repercussions for the UN's nuclear watchdog if European nations that launched a dispute mechanism against the Islamic republic act "unfairly".
Britain, France and Germany launched a process last week charging Iran with failing to observe the terms of the 2015 deal curtailing its nuclear programme, while Tehran accuses the bloc of inaction over US sanctions.
The EU three insisted they remained committed to the agreement, which has already been severely undermined by the US exit from it in 2018 and its reimposition of unilateral sanctions on key sectors of Iran's economy.
"What the three European countries did regarding Iran's nuclear issue... is unfortunate," parliamentary speaker Ali Larijani was quoted as saying by state news agency IRNA.
"We clearly announce that if Europe, for any reason, uses Article 37 of the nuclear agreement unfairly, then Iran will make a serious decision regarding cooperation with the agency," he said, referring to the International Atomic Energy Agency (IAEA).
Since May 2019, Iran has progressively scaled back some commitments under the agreement in response to the US sanctions and Europe's inability to circumvent them.
It has stressed, however, that they can be reversed if Tehran's interests are realised.
Iran's latest and final step in January entailed forgoing the limit on the number of machines used to make uranium more potent.
The 2015 nuclear deal -- known as the Joint Comprehensive Plan of Action (JCPOA) -- was struck in Vienna by Iran, the EU three, the United States, China and Russia.
It has a provision that allows a party to claim significant non-compliance by another party before a joint commission.
Articles 36 and 37 of the deal say 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.
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 air strike, and the admission by Tehran days later that it had accidentally shot down a Ukrainian airliner.
"The issue is not Iran's behaviour," said the parliamentary speaker.
"It is America's threats that have pushed a powerful European country to a humiliating and unjust" position, said Larijani.
Germany confirmed last week that the United States had been threatening to impose a 25-percent tariff on European cars if the bloc continued to back the nuclear deal.