The US has exempted three Iranian nuclear sites from sanctions, but warned that they will be under strict scrutiny, the State Department said.
The benefitted sites were those of Arak (central Iran), Bushehr (south) and Fordow, near the city of Qom that was built underground to protect against military attack.
The measure announced on Monday was "interim" and only allowed the continuation of the three non-proliferation projects "under the strictest scrutiny to ensure transparency and maintain constraints on Iran" Efe quoted the State Department as saying.
Early on Monday the US imposed its "toughest ever" sanctions on Iran that it had earlier lifted after the signing of the nuclear agreement in July, 2015 between Tehran and Russia, China, the UK, France, Germany and the US.
US President Donald Trump's administration in May reinstated all sanctions lifted under the 2015 nuclear deal, targeting both Iran and countries that trade with it in oil, financial transactions with its Central Bank and the country's port sector.