The move is intended to end Russian and European cooperation with Iran on the once-secret site, which was supposed to become a civilian research center under the 2015 agreement.

Secretary of State Mike Pompeo pointed to Iranian President Hassan Rouhani's announcement of renewed activity at Fordow -- a series of steps taken by Tehran as it presses European allies to make good on sanctions relief promised for compliance.

"Therefore the United States will terminate the sanctions waiver related to the nuclear facility at Fordow effective December 15, 2019," Pompeo told a news conference.

"The right amount of uranium enrichment for the world's largest state sponsor of terror is zero," Pompeo said.

"There is no legitimate reason for Iran to resume enrichment at this previously clandestine site. Iran should reverse its activity there immediately," he said.

President Donald Trump last year withdrew from the denuclearization accord and reimposed sweeping sanctions on Iran, aiming to impose economic pain and reduce the Shiite clerical regime's influence in Arab countries.

But the Trump administration has still granted waivers to let other nations implement the nuclear deal, which was negotiated under former president Barack Obama, without facing penalties in the United States.

Critics of Trump have said that the waivers prove that the administration sees the benefits of the deal, while hawkish Republicans have been pressing on Pompeo to end them.

Iran said that engineers at Fordow, an underground plant south at Tehran, began feeding uranium hexafluoride gas into mothballed enrichment centrifuges.

Britain, France, Germany and the European Union said Iran's decision was "inconsistent" with the nuclear deal.

The latest US step comes as Iran is rocked by violent protests sparked by the sudden decision to hike gas prices.

Pompeo again voiced solidarity with the protesters.

"The world's watching," he said.

"The Iranian people will enjoy a better future when their government begins to respect basic human rights, abandons its revolutionary posture and its destabilizing foreign policy in the region, and behaves simply like a normal nation," he said.