The US has rebuffed high-level pleas from the European Union to grant exemptions to European companies from its sanctions against Iran.
In a letter to European nations, Secretary of State Mike Pompeo said the US rejected the appeal because it wants to exert maximum pressure on Iran.
It said exemptions would only be made if they benefited US national security.
The EU fears that billions of dollars’ worth of trade could be jeopardised as a result of Washington’s new sanctions.
“We will seek to provide unprecedented financial pressure on the Iranian regime,” the letter, which was also signed by Treasury Secretary Steven Mnuchin, said, according to NBC news.
It added that the US is “not in a position to make exceptions to this policy except in very specific circumstances”.
The strict sanctions were imposed in May after President Donald Trump withdrew from the 2015 international agreement on Iran’s nuclear programme.