The United States said on Monday it will no longer waive sanctions stemming from operations at Iran's Fordow nuclear plant after Teheran resumed uranium enrichment at the underground site.

US Secretary of State Mike Pompeo made the statement in response to Iranian President Hassan Rouhani's announcement of renewed activity at Fordow - the latest step taken by Teheran as it presses European leaders to make good on sanctions relief promised for compliance with a 2015 multilateral accord.

"Therefore, the United States will terminate the sanctions waiver related to the nuclear facility at Fordow, effective on Dec 15, 2019," Pompeo said.

The waivers are among the last remaining components of the 2015 nuclear deal US President Donald Trump withdrew from last year.

However, it could be difficult for the cancellation to have a direct impact on Iran, said Zou Zhiqiang, a researcher at Shanghai International Studies University's Middle East Studies Institute.

"Iran no longer depends on outside support for its nuclear development after going through the US' comprehensive sanctions and extreme pressure.

"Under the current circumstances, Teheran will not retreat easily, and will continue to expand its nuclear development step by step," he said.

Zou added that ending the waivers could be more "harmful" to efforts to persuade Iran to return to compliance with the nuclear deal.

"The waivers were mainly for enterprises from other countries that participated in the transformation of Iran's nuclear facilities under the framework of the 2015 accord. The cancellation will only negatively affect such cooperation," he said.

The International Atomic Energy Agency, or IAEA, the United Nations atomic watchdog, and Iran itself reported earlier this month that Teheran was again enriching uranium at the sensitive Fordow site. Iran had hidden the site from UN nonproliferation inspectors until its exposure in 2009. Under the 2015 agreement signed with world powers, Iran had committed to transforming it into a civilian research center.

The IAEA said on 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 uraniumwas 131.5 tons, above the 130-ton limit, said a spokesperson for the agency in Vienna.

Early this month, Iran said it began feeding uranium hexafluoride gas into mothballed enrichment centrifuges at Fordow.

AFP and Reuters contributed to this story.