According to state media, Iran has started enriching uranium to 60 percent purity at its underground Fordow nuclear site. It is seen as a response to the U.N. nuclear watchdog’s demand for more cooperation from Tehran.

Iran is already enriching uranium to a similar level of 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 IAEA’s latest resolution.

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

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

Tehran was reported in June to be advancing 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.