The US on Monday has rejected pleas from the EU to grant exemptions to European firms from the sanctions on Iran that will start to be reinstated next month.
Secretary of State Mike Pompeo and Treasury Secretary Steven Mnuchin wrote in a letter that they wanted to exert "unprecedented" pressure on Tehran.
Waivers will only be given on national security or humanitarian grounds, reports BBC.
US President Donald Trump ordered that the sanctions be re-imposed in May after abandoning a nuclear deal with Iran.
The UK, France and Germany - which were also parties to the 2015 accord, along with Russia and China - pledged to continue abiding by their commitments.
They said they would work on measures to counter the effects of the sanctions, but major European companies like Peugeot and Total have since cut back their involvement in Iran in order not to risk their business prospects in the US.
Iran has said it will stay in the deal if it still receives the economic benefits.