"Iran enriching uranium to 20 percent at Fordow is a clear attempt to increase its campaign of nuclear extortion, an attempt that will continue to fail," a State Department spokesperson said.

"We have confidence that the IAEA will monitor and report on any new Iranian nuclear activities," the spokesperson said, referring to the International Atomic Energy Agency.

The UN watchdog confirmed that Iran stepped up the enrichment at its underground Fordow facility in Tehran's biggest breach yet of a 2015 nuclear deal negotiated under former president Barack Obama.

Trump withdrew from the deal and reimposed crippling sanctions in violation of the accord.

Iranian Foreign Minister Mohammad Javad Zarif said after the enrichment announcement that his country would return to full compliance if all countries do -- a signal to Biden to remove sanctions.