In a surprise move, U.S. President Barack Obama declined to sign a 10-year extension of legislation governing sanctions against Iran but allowed it to become law anyway.
Obama faced a deadline of midnight on December 14 to sign the bill extending the Iran Sanctions Act, which allows the United States to impose sanctions on companies for doing business with Iran.
He did not sign it but also did not veto it, and White House spokesman Josh Earnest said that it "is becoming law without the president's signature" under a complex U.S. legislative rule.
Iran had urged Obama not to sign the extension and vowed to respond if it was enacted, saying that it violates the 2015 international agreement curbing Tehran's nuclear program in exchange for sanctions relief.
But Obama has said the legislation would make no difference to the agreement because the White House, for the rest of his term, will continue to suspend all the sanctions linked to Iran's nuclear program.
President-elect Donald Trump, who takes office in January, has been sharply critical of the nuclear deal, which is designed to delay any Iranian effort to acquire nuclear arms.
A senior Russian diplomat said on July 15 that it would be "inexcusable" to let the deal negotiated by Iran and six global powers -- the United States, Russia, China, Britain, France, and Germany -- be scuttled.
"This deal must be protected," said Mikhail Ulyanov, the Russian Foreign Ministry's point man for arms control and nonproliferation.