Iran’s Islamic Revolution Guards Corps (IRGC) said on Thursday that its naval forces had seized two vessels carrying smuggled fuel near Farsi Island in the Persian Gulf.

In a statement published by its official outlet Sepah News, the IRGC said the vessels were seized under a judicial order and transferred to the southern port city of Bushehr, The Caspian Post reports.

Fifteen foreign crew members aboard the ships were detained and handed over to Iran’s judicial authorities for legal proceedings.

The IRGC added that the two vessels were carrying more than one million liters of smuggled diesel fuel. According to the statement, the ships were part of a fuel-smuggling network active in recent months and were identified and confiscated following monitoring and tracking by the IRGC Navy’s intelligence units.

Fuel smuggling remains a persistent problem in Iran, which has some of the world’s lowest fuel prices. The wide price gap has created a lucrative market for smugglers who transport Iranian fuel to neighboring countries where prices are much higher. Iranian officials estimate that the state loses billions of U.S. dollars each year due to the illicit trade.