The US is considering action in response to Iran's shipment of fuel to crisis-stricken Venezuela, a senior official in President Donald Trump's administration said on Thursday.
The US has a "high degree of certainty" that Venezuelan President Nicolas Maduro's government is paying Iran tonnes of gold for the fuel, the official said.
"It is not only unwelcome by the United States but it's unwelcome by the region, and we're looking at measures that can be taken," he said.
The oil sectors of Iran and Venezuela, members of Opec and deeply at odds with the US, are both under severe American sanctions.
The official said response options would be presented to Mr Trump.
At least one tanker carrying fuel loaded at an Iranian port has set sail for Venezuela, vessel tracking data from Refinitiv Eikon showed on Wednesday.
The shipload could help to ease an acute lack of petrol in the South American country.
The Iranian-flagged medium tanker Clavel on Wednesday passed the Suez Canal after loading fuel at the end of March at Iran's Bandar Abbas port, the data showed.
Venezuela is in the middle of an economic collapse under the socialist Mr Maduro.
It produces crude oil and has refining capacity for 1.3 barrels a day, but its infrastructure has been crippled by under-investment and lack of maintenance.
The shipment marks the latest sign of co-operation between Iran and Venezuela.
Starting last month, flights from Tehran have taken material to Venezuela to help it restart the catalytic cracking unit at its 310,000 barrel-per-day Cardon refinery, drawing US condemnation.
Last year, the US imposed sanctions on the state-owned Petroleos de Venezuela as part of Trump administration efforts to remove Mr Maduro, whose 2018 re-election was considered a sham by most western countries.
The US and dozens of other nations recognised opposition leader Juan Guaido as interim president last year.
But Mr Maduro, who calls Mr Guaido a US puppet, remains in power, backed by Venezuela's military and Russia, China, Cuba and Iran.
It has become a growing source of frustration for Mr Trump, US officials say.
The US also imposed punishing sanctions on Iran aimed at containing its influence in the Middle East, after Mr Trump pulled out of a nuclear deal between Tehran and world powers.
Four other vessels of the same size as the Clavel, all flagged in Iran and loaded with fuel at or near Bandar Abbas, are about to cross the Atlantic Ocean after passing Suez.
They have not yet set their final destinations, data showed.
One of them, the Fortune, appears on a list of tankers scheduled to enter a Venezuelan port, a source said.
Opposition politicians also said they had received information that all five tankers were heading to Venezuela.
They are carrying petrol, said Samir Madani, co-founder of TankerTrackers.com, which tracks oil shipments and storage.
Mr Madani said the vessels loaded at Berths 1 and 2 at the Shahid Rajaee port at Bandar Abbas, according to his service's satellite imagery.
US Secretary of State Mike Pompeo last month called on countries to deny airspace to Iranian airline Mahan Air, which is under US sanctions.
Mr Pompeo said Mahan delivered cargoes of "unknown support" to the Venezuelan government.