Iran has started enriching uranium to 60 percent purity at its underground Fordow nuclear site, reported state media on Tuesday, which described the action as a response to the UN nuclear watchdog's demand for more cooperation from Tehran.

Iran is already enriching uranium to up to 60 percent purity elsewhere, well below the roughly 90 percent needed for weapons grade material but above the 20 percent it produced before a 2015 agreement with major powers to cap enrichment at 3.67 percent.

"In a letter to the International Atomic Energy Agency (IAEA), Iran has informed the agency that it has started enriching uranium to 60 percent purity at Fordow site," the semi-official ISNA news agency reported, adding that it was a "strong response" to the agency's latest resolution.

The IAEA 35-nation Board of Governors on Thursday passed a resolution ordering Iran to cooperate urgently with the agency's investigation into uranium traces found at three undeclared sites, diplomats at the closed-door vote said.

Iran's SNN network said Tehran will also be building a new set of centrifuges at its Natanz and Fordow nuclear sites.

In June, Reuters reported that Tehran was escalating its uranium enrichment further by preparing to use advanced IR-6 centrifuges at the Fordow site, which can easily switch between enrichment levels.

The IAEA resolution is the second this year targeting Iran over the investigation, which has become an obstacle to talks on reviving the 2015 Iran nuclear deal because Iran has demanded the probe be ended.

Iran's foreign ministry on Monday dismissed the resolution as "politically motivated."

(Cover: File photo released on November 5, 2019 by the Atomic Energy Organization of Iran shows centrifuge machines in the Natanz uranium enrichment facility in central Iran. /CFP)