Piling pressure as protests hit Iran, the move is intended to discourage work led by Russia's state-owned nuclear company Rosatom at the once-secret site, which was supposed to be transformed into 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 -- one of a series of steps taken by Tehran as it presses Europeans 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 inflict 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 proved the administration saw the benefits of the deal, which sharply expanded access for international inspectors. Iran had been in compliance before the US withdrawal.
US waivers remain in place for other nuclear sites including Bushehr, a nuclear power plant off the Gulf coast being developed with Russia under safeguards. Hawkish Republicans have been pressing on Pompeo to end the waivers. Senator Tom Cotton praised the decision as he pointed to the weekend protests in Iran over gas price hikes.
"The mullahs are playing with atoms in an underground bunker at Fordow instead of helping their own people, who are demonstrating in the street," he tweeted.
- Latest step by Iran -
Iran, which insists its nuclear program is exclusively peaceful, has taken a series of measures that fall well short of developing a nuclear weapon but are meant to put pressure on European powers. The UN's nuclear watchdog said Monday that Iran's stock of heavy water for reactors had crossed the level agreed upon in the 2015 accord for the first time.
Iran's stock of heavy water -- which can be used to produce plutonium for nuclear weapons as an alternative to enriched uranium -- was 131.5 tonnes, above the 130-tonne limit, said a spokesperson for the agency in Vienna. At Fordow, Iran said it 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. Iran has accused European nations of hypocrisy over the nuclear deal and also attacked the United States for its statements of support for protesters.
Major roads have been blocked, banks torched and shops looted since Friday's abrupt hike in gas prices. Pompeo again voiced solidarity with the protesters, saying: "The world's watching."
"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.