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.

The president, who had previously been expected to sign the measure, symbolically let slide a midnight deadline to ink his name on the legislation - which he has called unnecessary - meaning the 10-year sanctions renewal will 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 program.

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, criticising the US move as a breach of the nuclear accord.

-AFP