The secret transfers usually take place at night to evade detection by regional coast guards. The ships anchor in the Persian Gulf just outside the territorial limits of the United Arab Emirates, and then, individually, small boats carrying smuggled Iranian diesel shift their loads to the waiting vessels, according to seafarers who have witnessed the trade.

“It is a big chain, with fishing boats sailing up to give diesel to a waiting tanker. It takes four to five days because boats come one by one,” said a 27-year-old Indian seafarer, who spoke on the condition of anonymity for fear of reprisals. He said he had been employed by a Dubai-based shipping company that smuggled Iranian fuel to Somalia.

His description of these illicit operations, which accelerated when the United States reimposed sanctions on Iranian oil exports after President Donald Trump withdrew from the Iranian nuclear deal in 2018, is one of five eyewitness accounts provided by Indian nationals who say they worked on vessels involved in the clandestine commerce. While smuggling of Iranian petroleum products has been documented previously and drawn a U.S. rebuke, these seafarers offered a rare inside look at how these activities are carried out.

The tankers always anchor in the international waters that separate Iran and the UAE, recounted a 28-year-old Indian seafarer, who said he worked for two companies involved in smuggling Iranian diesel between 2016 and 2020.

“UAE territorial waters end after 12 miles, so Iranian ships come as close as 14 to 20 miles to the UAE,” he said. “They switch off their AIS [automatic identification system] so that they can’t be tracked. If they see the UAE coast guard, they stop the operation and run away.”

In addition to the nighttime transfers at sea, Iranian diesel bound for international markets is exported on tankers setting sail from Iran with the origin of the shipment forged to make it look as though it came from Iraq or the UAE, according to a third seafarer and three experts in security and energy affairs.

Because of the profit margins, this trade was highly lucrative even before the United States pulled out of the nuclear deal. Iran has some of the world’s cheapest fuel prices thanks to very low production costs, heavy government subsidies and a weak currency. But the reimposed economic sanctions have given this business a further boost as smugglers seek to evade restrictions on Iranian oil exports. Those sanctions are now a focus of discussions in Vienna, where Iran and world powers have resumed negotiations aimed at reviving the nuclear accord.

“The transport of sanctioned Iranian [petroleum] products happens on a weekly basis,” said Cormac Mc Garry, associate director of Control Risks, a consultancy. “There are financial drivers and the demand, so Iran will find a way around sanctions. And its policy is to keep that an absolute secret. They don’t reveal how they do it.”