The ship was carrying 700,000 liters [185,000 gallons] of fuel, according to state TV and the semi-official Fars news agency. Seven crew members were reportedly detained during the vessel's seizure.
The ship was seized on Wednesday near Farsi Island, where an Iranian Guard Navy base is located, the news outlet reported. The island sits in the Persian Gulf between Saudi Arabia and Iran, north of the Strait of Hormuz.
"This foreign vessel had received the fuel from other ships and was transferring it to Persian Gulf Arab states," Gen. Ramazan Zirahi, a Guard commander, was quoted as saying.
Further information on the vessel, and the nationality of crew members, was not immediately clear.
The seizure marked the third such incident involving a commercial vessel in recent weeks — and the second accused of smuggling fuel.
The U.S. Fifth Fleet, based in Bahrain, told The Associated Press it did not have information to confirm the reports. Maritime tracking experts also said they did not have any immediate information about the incident or the vessel.
On July 18, the paramilitary force seized a United Arab Emirates-based oil tanker, the Panamanian-flagged MT Riah, for allegedly smuggling some 264,000 gallons of fuel from Iranian smugglers to foreign customers.
The following week, the Guard's naval forces seized a British-flagged vessel in the Gulf in what some Iranian officials suggested was retaliation for the seizure of an Iranian oil tanker in a British Royal Navy operation off Gibraltar. The U.K. says the Iranian oil tanker was suspected of violating European Union sanctions on oil shipments to Syria. Iran denies the ship was bound for Syria but has not disclosed its destination.
The seizure comes amid heightened tensions between the U.S. and Iran amid the breakdown of the nuclear deal that led the regime to exceed the threshold of low-enriched uranium stockpile as agreed upon in the 2015 accord.
The Associated Press contributed to this report.