Iran has followed through on a threat to accelerate its production of enriched uranium, the head of the UN's atomic watchdog said on Monday, departing from his usual guarded language to say he was worried about increasing tension, Al Jazeera reports.

The assessment comes at a time of sharply increased US-Iranian confrontation, a year after Washington abandoned an international agreement that imposed curbs on Tehran's nuclear programme in exchange for the lifting of financial sanctions.

Washington tightened sanctions on Iran from the start of May, ordering all countries and companies to halt any imports of Iranian oil or be banished from the global financial system. It also dispatched extra troops to the region to counter unspecified threats from Iran.

Iran responded with a threat to increase its enrichment of uranium, saying it was up to European countries who still support the nuclear deal to save it by finding ways to ensure Tehran receives the economic benefits it was promised.

IAEA chief Yukiya Amano, whose agency is responsible for monitoring Iranian compliance with the nuclear deal, said Iran was now producing more enriched uranium than before, but it was not clear when it might reach the stockpile limit of 300 kg set in the pact.

"Yes, [the] production rate is increasing," he told a news conference on Monday when asked if enriched uranium production had accelerated since the agency's last quarterly report, which found Iran compliant with the nuclear deal as of May 20.

He declined to quantify the increase.

"I am worried about increasing tensions over the Iranian nuclear issue," he said, adding he hoped "that ways can be found to reduce current tensions through dialogue. It is essential that Iran fully implements its nuclear-related commitments".