The measures also aim to save the agreement meant to curb Iran's nuclear ambitions after the US abandoned the pact. The EU insists the deal is important for global security and is trying to keep economic and financial supply lines to Tehran open.
The mechanism stops European companies from complying with the US sanctions unless they have authorisation from the Commission. National governments could impose "effective, proportionate and dissuasive penalties" on any of their companies that cave in.
In a statement, the foreign ministers of Britain, France and Germany the European guarantors of the Iran nuclear deal together with EU foreign policy chief Federica Mogherini said, "We deeply regret the re-imposition of sanctions by the U.S."
They said the deal "is working and delivering on its goal" and that "we are determined to protect European economic operators engaged in legitimate business with Iran."
US President Donald Trump decided in May to pull out of the UN Security Council-sanctioned agreement, saying it did nothing to stop Iran developing missiles or destabilizing the Middle East and Gulf regions.
The Europeans argue that it was only ever meant to curb the development of nuclear weapons.
A first set of US sanctions that had been eased by the Obama administration under the terms of the landmark 2015 deal take effect again late today. Those sanctions target Iran's automotive sector as well as gold and other metals, but EU officials say it remains unclear exactly what impact they will have on European companies.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)