According to the semi-official Fars news agency, Iran's Islamic Revolutionary Guard Corps (IRGC) forces ambushed the tanker on Sunday, in the latest incident in a series of dramatic developments as tensions increase between the US and Iran over the nuclear deal between Tehran and world powers.
Press TV said that the oil tanker was intercepted south of Iran's Larak Island in the Strait of Hormuz.
IRGC naval forces, which were patrolling the waters on an anti-smuggling mission, acted against the vessel in a "surprise" operation upon ascertaining the nature of its cargo and securing the required legal approval from Iranian authorities, according to the state TV.
The state media said that the ship had loaded the fuel from Iranian dhows and was about to hand it over to other foreign vessels in farther waters. The seized vessel or nationalities of the crew were not identified.
The vessel, which had 12 foreign crew members aboard at the time of the mission, is capable of carrying two million liters of fuel, Iranian reports said.
It was not clear whether the seized tanker was the same that Tehran had claimed to have assisted earlier this week after receiving a distress call.
The incident comes after a brief standoff last week between the British Navy and IRGC in the Strait of Hormuz.
The UK government said a Navy frigate had to warn away the paramilitary vessels from disrupting the passage of a British oil tanker through the strait.
It also comes after British marines helped seize Iranian supertanker Grace 1 off Gibraltar, a British overseas territory.
Britain has since said it would facilitate the release of the tanker if Tehran can provide guarantees the vessel "would not be going to Syria".
( With inputs from IANS )