Secretary of State Mike Pompeo also confirmed the US’ support for Venezuelan opposition leader Juan Guaido. Mr Pompeo said at a news briefing that the ships carried about 1.5 million barrels of Iranian gasoline and associated components.
Additionally, he issued a warning to mariners against striking trade deals with Venezuelan President Nicolas Maduro’s government.
Washington is actively seeking to overthrow Mr Maduro.
In a later statement, Mr Pompeo said: “As a result of today’s sanctions, these captains’ assets will be blocked.
“Their careers and prospects will suffer from this designation.
“Mariners who are considering work with Iran and Venezuela should understand that aiding these oppressive regimes is simply not worth the risk. Individuals and entities will face consequences from the United States if they do business with the Iranian regime, Maduro, or his cronies.”
“We will continue to support the National Assembly, interim President Guaido, and the Venezuelan people in their quest to restore democracy,” Mr Pompeo said to reporters.
President Donald Trump’s administration is pursuing Mr Maduro’s ouster as well as stopping Iran’s energy trade.
The administration threatened admonitions and cautioned ports, shipping companies and insurers against working with the tankers.
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Iranian Foreign Ministry spokesman Abbas Mousavi wrote in a tweet that the US’s move showed the lack of success of its pressure campaign.
Mr Mousavi warned that Iran and Venezuela “remain steadfast in countering unlawful American sanctions”.
Iran has five shipments since April to Mr Maduro’s government of oil-deprived Venezuela.
The tankers have not had much effect in Venezuela’s profound lack fuel, with hours-long queues still in place at petrol stations.
In an interview with news platform Axios released on Sunday, Trump minimised his January 2019 move to acknowledge Mr Guaido, speaker of the opposition-held National Assembly, as Venezuela’s true leader.