"With a capacity of two million barrels and 12 foreign crew on board, the vessel was en route to deliver contraband fuel received from Iranian boats to foreign ships in farther regions when it was intercepted," the force said.
The announcement comes two days after Iran said it had come to the aid of a foreign tanker after receiving a distress call -- making no mention of the vessel being seized.
"Iranian forces approached it and using a tugboat brought it into Iranian waters for necessary repairs," foreign ministry spokesman Abbas Mousavi said.
TankerTrackers reported that the Panamanian-flagged tanker Riah, used in the Strait of Hormuz "for fuelling other vessels", had crossed into Iranian waters on Sunday.
According to the online oil shipment tracking service, at that point the tanker's automatic identification system stopped sending signals.
Watch: Iran says UK's seizure of oil tanker will not go unanswered