President Barack Obama allowed US sanctions against Iran to be renewed on Thursday, but in a surprise move declined to actually sign the legislation that brings the sanctions into force.

"The extension of the Iran Sanctions Act is becoming law without the president's signature," White House spokesman Josh Earnest said in a statement.

In an apparent reversal, the president, who had previously been expected to sign, symbolically let slide a midnight deadline for the legislation - which he has called unnecessary - meaning the 10-year sanctions renewal would automatically become law.

Under the Iran nuclear deal signed in July 2015, world powers agreed to lift international sanctions in exchange for curbs to Iran's nuclear program.

Obama has said that passage of the US sanctions renewal would make no difference to the agreement because the White House will continue to suspend all the sanctions linked to Iran's nuclear programme.

The language in the nuclear agreement makes it unclear whether renewing the sanctions - and keeping the nuclear ones suspended - amounts to a violation.

"This administration has made clear that an extension of the Iran Sanctions Act, while unnecessary, is entirely consistent with our commitments in the Joint Comprehensive Plan of Action (JCPOA)," Earnest said, referring to the nuclear deal by its formal name.

Iran's President Hassan Rouhani on Tuesday ordered the country's scientists to start work on nuclear-powered ships in response to the expected renewal of sanctions, criticizing the US move as a breach of the nuclear accord.

International analysts said the announcement was likely just a bluff, since it would be an extremely costly effort for little strategic gain.

US Secretary of State John Kerry said Thursday that continued implementation of the Iran nuclear deal remained "a top strategic objective" for the United States.

Kerry echoed the White House's assertion that the legislation was unnecessary, stating that with or without the renewed sanctions, the United States would still be able to address any Iranian breach of the nuclear deal or snap back sanctions should Iran fall short of its commitments.

"The administration has, and continues to use, all of the necessary authorities to waive the relevant sanctions, to enforce those that are outside the scope of the JCPOA, and to re-impose sanctions if necessary in the event that Iran should fail to perform its commitments under the JCPOA," Kerry said in a statement.