The International Atomic Energy Agency (IAEA) has a "key role... in monitoring and verifying the implementation by Iran of the nuclear commitments" under the 2015 accord with world powers, Commission spokeswoman Maja Kocijancic told a media briefing.

"Our commitment to the nuclear deal depends on full compliance by Iran," she said, adding that the EU based its assessment of that "on the reports done by the International Atomic Energy Agency".

Iran this week announced its latest step away from the accord in reaction to intensifying sanctions pressure from the US, which exited the deal last year.

Tehran's declaration that it was to resume nuclear research and development was the third cut to its commitments since July, sparking fears the accord is crumbling despite European efforts to keep it going.

The IAEA in its latest report, on August 30, said it continues to verify compliance through cameras and on-site inspections. But in an apparent hint at worries about access it said "ongoing interactions ... require full and timely cooperation by Iran".

Iran sent a letter to the EU's top foreign policy official Federica Mogherini detailing its latest cut to the terms of the accord.

Kocijancic said that "we note with great concern that announcement made by Iran in the letter".

She reiterated the Commission's exhortation that Iran "reverse all activities that are inconsistent with its commitments... and to refrain from any further measures that undermine the preservation and full implementation of the nuclear deal."

She also noted that an initiative by European powers Britain, Germany and France to provide a financial channel for Iran immune from US sanctions -- a "special purpose vehicle" known as INSTEX -- had started to process payments.

Iran argues that Europe is not doing enough to shield its vital oil sales from the US sanctions and that it is therefore justified in pulling back, bit-by-bit, from the nuclear accord.