Demanding EU allies follow suit, US reimposes sanctions on Tehran after the Trump administration's unilateral weekend declaration that all UN penalties that were eased under the 2015 nuclear deal have been restored.
The United States has imposed sanctions on Iran's Defence Ministry and Venezuelan President Nicolas Maduro under a contested UN authority and demanded that European powers follow suit.
President Donald Trump's administration named 27 people or entities that it said would be subject to UN sanctions, but the world body itself says that the decision is not up to Washington.
"The United States has now restored UN sanctions on Iran," Trump said in a statement.
"My actions today send a clear message to the Iranian regime and those in the international community who refuse to stand up to Iran."
READ MORE: US expands scope of sanctions on Iran
Unilateral action by US
The Trump administration argues that it is enforcing a UN arms embargo that Iran has violated, including through an attack on Saudi oil facilities.
But it is using a UN resolution that approved a 2015 nuclear accord with Iran negotiated by former president Barack Obama.
Trump pulled out of the resolution with fanfare in 2018 but argues that the United States is still a "participant" in the accord as it was listed in the 2015 resolution.
The legal argument has been rejected by virtually all nations on the UN Security Council, with European allies of the United States saying that the priority is to salvage a peaceful solution on Iran's nuclear programme.
Enforce sanctions – US tells EU powers
"We have made it very clear that every member state in the United Nations has a responsibility to enforce the sanctions," US Secretary of State Mike Pompeo told reporters on Monday when asked about European opposition.
"That certainly includes the United Kingdom, France, and Germany. We will have every expectation that those nations enforce these sanctions," he said.
The individuals listed under the "UN" sanctions are mostly already targeted by the United States and include Iran's Defence Ministry and its Atomic Energy Organisation.
The United States has separately been seeking to oust Venezuelan President Nicolas Maduro, who has increasingly sought cooperation with Iran on the oil sector.
The State Department said it was again imposing sanctions on Maduro under the executive order from Trump that is based on the UN resolution, pointing to defence transactions between Iran and the leftist Venezuelan leader.
"For nearly two years, corrupt officials in Tehran have worked with the illegitimate regime in Venezuela to flout the UN arms embargo," Pompeo said.
"Our actions today are a warning that should be heard worldwide."
The United States already imposed sweeping sanctions on Iran in 2018 when Trump withdrew from the nuclear accord, which was meant to offer economic relief to Iran – seen at the time by UN inspectors as in compliance.