Iran said earlier Monday it had "crossed the 300-kilogramme limit" set under a 2015 nuclear deal that was unilaterally abandoned by US President Donald Trump last year.

"(This) of course is a cause for regret but one mustn't dramatise the situation," Russian deputy foreign minister Sergei Ryabkov said in comments reported by news agencies.

"It should be understood as the natural consequence of the events which have gone before," he said.

Ryabkov denounced "unprecedented pressure" from the United States but called on Tehran to behave "responsibly".

Moscow is a close ally of Tehran and has previously called on European signatories of the nuclear agreement to respect the deal despite the US pullout.

When the United States withdrew from the nuclear deal last year it reimposed biting sanctions on Iran's crucial oil exports and financial transactions as well as other sectors.

Tehran, which has sought to pressure the remaining parties to save the deal, last month announced it would no longer respect the limit set on its enriched uranium and heavy water stockpiles.