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In an unexpected reversal, President Barack Obama declined to sign a renewal of sanctions against Iran but let it become law anyway, in an apparent bid to alleviate Tehran's concerns that the U.S. is backsliding on the nuclear deal.

Although the White House had said that Obama was expected to sign the 10-year-renewal, the midnight deadline came and went Thursday with no approval from the president. White House press secretary Josh Earnest said Obama had decided to let it become law without his signature.

"The administration has, and continues to use, all of the necessary authorities to waive the relevant sanctions" lifted as part of the nuclear deal, Earnest said in a statement.

Under the Constitution, the president has 10 days after Congress passes a bill to sign it, veto it or let it become law with no signature if Congress is still in session. Although lawmakers have returned home for the holidays, Congress technically has not adjourned and is holding "pro-forma" sessions this week.

Though Obama's move doesn't prevent the sanctions renewal from entering force, it marked a symbolic attempt by the president to demonstrate disapproval for lawmakers' actions. The White House has argued that the renewal is unnecessary because the administration retains other authorities to punish Iran, if necessary, and has expressed concern that the renewal may undermine the nuclear deal.

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Iran's president had vowed to respond if the sanctions were renewed, arguing they violate the nuclear deal between Iran and world powers, which eased sanctions in exchange for curbs on Iran's nuclear program. On Tuesday, Iran's president ordered up plans to build nuclear-powered ships and to formally accuse the U.S. of violating the terms of the deal.

The Obama administration stressed that Iran would be unaffected by the renewal, as long as it continues honoring the nuclear deal. Secretary of State John Kerry said he'd told his Iranian counterpart that "to ensure maximum clarity," he'd issued new, redundant waivers exempting Iran from sanctions lifted under the deal.

"Extension of the Iran Sanctions Act does not affect in any way the scope of the sanctions relief Iran is receiving under the deal or the ability of companies to do business in Iran consistent with the JCPOA," Kerry said, using an acronym for the nuclear deal.

President-elect Donald Trump has been sharply critical of the nuclear deal and has threatened to try to renegotiate it, but has been vague about whether he'd pull the U.S. out of it.