"Maximum pressure on the Iranian regime will continue until its leaders alter their course of action," said a statement from White House Press Secretary Stephanie Grisham.

"The United States and its allies will never allow Iran to develop nuclear weapons," it said, calling it "a mistake under the Iran nuclear deal to allow Iran to enrich uranium at any level."

"We must restore the longstanding nonproliferation standard of no enrichment for Iran," the statement said.

The US withdrew from the 2015 nuclear deal with Iran last year, imposing punishing sanctions, and relations have sharply deteriorated since.

Tehran, which has sought to pressure the remaining parties to save the deal, on May 8 announced it would no longer respect the limit set on its enriched uranium and heavy water stockpiles.

It also threatened to go further and abandon more nuclear commitments unless the remaining partners -- Britain, China, France, Germany and Russia -- helped it to circumvent sanctions, especially to sell its oil.

Washington has blamed Iran for a series of attacks on tanker ships and Tehran shot down an American surveillance drone last month, raising fears of an unintended slide toward conflict that both sides have said they want to avoid.