"Iran has no credible reason to expand its uranium enrichment program, at the Fordow facility or elsewhere, other than a clear attempt at nuclear extortion that will only deepen its political and economic isolation," a State Department spokesperson said.

"We will continue to impose maximum pressure on the regime until it abandons its destabilizing behavior, including proliferation-sensitive work."

President Hassan Rouhani said Iran would resume enrichment at the plant near the Shiite holy city of Qom that was suspended under a 2015 nuclear accord with the United States and five other powers.

It was the latest action by Iran to seek tangible benefits from the deal, from which the United States withdrew as President Donald Trump imposed sweeping sanctions aimed at reducing Tehran's regional role.

The State Department said it would await verification by the International Atomic Energy Agency, the UN nuclear watchdog which stepped up inspection under the 2015 accord.