The US government will impose sanctions on the captains of five Iranian ships that delivered fuel to Venezuela last month, US Secretary of State Mike Pompeo said on Wednesday.
The US has also warned of a return of UN sanctions on Iran if the Security Council fails to extend an arms embargo expiring in October.
The captains' sanctions follow the delivery of 1.5 million barrels of fuel from Iran to Venezuela, in defiance of an earlier ban by Washington.
Mr Pompeo said the US would continue to apply pressure to the regimes in Caracas and Tehran.
The US Treasury Department said the ships, operated by Iran Shipping Lines and National Iranian Tanker Company, had previously been cited in sanctions for having delivered Iranian petrol to Venezuela.
The US was the first of more than 50 nations to back Venezuelan President Nicolas Maduro’s political rival, Juan Guaido.
Mr Guaido, as head of the opposition-led Congress, claimed he was the rightful leader in early 2019 because he said Mr Maduro’s re-election was invalid.
The department said the captains of Iranian-flagged tankers Clavel, Petunia, Fortune, Forest and Faxon had been added to the Office of Foreign Assets Control's specially designated national and blocked persons list.
They were identified as Ali Kenarsari, Mohsen Gohardehi, Alireza Rahnavard, Reza Vaziri and Hamidreza Zadeh.
The US Treasury expects that more shipments will be sent from Iran to Venezuela.
“The Iranian regime has sent five tankers carrying over 1.5 million barrels of gasoline [petrol] and gasoline components to Venezuela, with plans to continue gasoline sales to the brutal and corrupt Maduro regime in the months to come,” it said.
US Secretary of Treasury Steven Mnuchin said: “The Iranian regime’s support to the authoritarian and corrupt regime in Venezuela is unacceptable, and the administration will continue to use its authorities to disrupt it.”
The shipments were delivered almost a month ago.
US President Donald Trump on Sunday told Axios that he would not rule out speaking to Mr Maduro in person.
“I’m never opposed to meetings, you know, rarely opposed to meetings," Mr Trump said.
"I always say you lose very little with meetings. But at this moment, I’ve turned them down."
After a backlash, he reversed his position in a tweet on Monday.
“My admin has always stood on the side of freedom and liberty, and against the oppressive Maduro regime," he wrote.
"I would only meet with Maduro to discuss one thing: a peaceful exit from power.”
But Democratic presidential nominee Joe Biden plans to use the about-turn to appeal to Venezuelan voters in Florida, many of whom oppose Mr Maduro.
On Wednesday, Mr Pompeo warned of a return of UN sanctions on Iran if the Security Council failed to extend an arms embargo expiring in October.
The US has put forward a resolution to extend the embargo on conventional weapons but faces opposition from Russia and China, which stand to win contracts with Tehran.
"Our focus now is to work with the Security Council to pass this resolution," Mr Pompeo said.
"But in the event it doesn't happen, we remind the world that the Obama administration's officials said very clearly that the United States has the unilateral ability to snap back sanctions into place."
The five-year ban on selling conventional arms was part of a 2015 resolution in support of a nuclear deal between Iran and world powers, negotiated by former president Barack Obama and his secretary of state, John Kerry.
Mr Trump withdrew the US from the deal in 2018.