Iran says it seized a "foreign tanker" and its 12 crew on Sunday for smuggling fuel in the Gulf.
Iran's state media published footage of Iranian speedboats circling around the Panamanian-flagged Riah tanker.
In a separate development, US President Donald Trump said the US Navy downed an Iranian drone in the Strait of Hormuz.
He said the USS Boxer "took defensive action" after the drone had come within about 1,000 yards (914m) of the vessel.
Iran has so far made no public comments on the drone.
Relations between the US and Iran are badly strained, with the US blaming Iran for attacks on oil tankers in the Gulf. Iran denies the allegations.
What did Iran say about the tanker?
The Revolutionary Guards' Sepah News site said on Thursday that the alleged fuel-smuggling ship was seized on Sunday during naval patrols aimed at "discovering and confronting organised smuggling".
The ship had a two-million-litre capacity, but was carrying one million litres of fuel when it was seized south of Iran's Larak Island, the website said.