Iran's Revolutionary Guards said Thursday they had detained a "foreign tanker" and its crew for allegedly smuggling fuel in the Gulf.

The tanker was seized on Sunday "south of the (Iranian) island of Larak" in the strategic Strait of Hormuz, the force's Sepahnews website said.

The Guards did not detail the name or provenance of the vessel.

"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.

The incident comes amid soaring tensions in the Gulf, with US President Donald Trump calling off air strikes against Iran at the last minute after Tehran downed an American drone.

Washington has blamed Tehran for a series of tanker attacks in recent months