Iranian state TV aired footage of a vessel called “Riah” that it said was seized by Iran’s elite Revolutionary Guards for smuggling fuel.
Iran played down the seizure of the ship, which it said was a small vessel that was smuggling oil.
The Panamanian-flagged oil tanker MT Riah disappeared off trackers in Iranian territorial waters days ago.
“We do this (inspecting ships) every day. These are people who smuggle our oil,” Iran's Press TV quoted Iranian Foreign Minister Mohammad Javad Zarif as saying, adding: “It was a small ship used to smuggle 1 million litres - not 1 million barrels - of crude oil.”
Earlier on Thursday, the United States demanded Iran immediately release a vessel it seized in the Gulf, and a US military commander in the region said the United States would work “aggressively” to ensure free passage of vessels through the vital waterway.