Iran has seized an oil tanker it claimed was carrying 1 million liters of "smuggled fuel," state news agency Press TV said on Thursday.
The semi-official Fars news agency said Islamic Revolutionary Guard Corps (IRGC) forces ambushed the tanker, carrying 12 people on board, on Sunday.
The IRGC said it had initially responded to distress calls from a ship on Sunday but when they searched it, they discovered it was a smuggling operation, according to Iranian state media.
Citing an IRGC statement, Fars reported that the ship -- which has a capacity of 2 million liters -- is a foreign tanker and was seized in an area south of Larak, a small island in the Strait of Hormuz.
Fars added that the ship was carrying fuel smuggled to it on Iranian dhows, or small boats.
In a statement on Thursday, a US State Department official said the US "strongly condemns the Islamic Revolutionary Guard Corps Navy's continued harassment of vessels and interference with safe passage in and around the Strait of Hormuz."
"Iran must cease this illicit activity and release the reportedly seized crew and vessel immediately," they said, adding that the US will "continue to work closely with our allies and partners to ensure the Iranian regime's extortion tactics and malign activities do not further disrupt maritime security and global commerce."
The ship's seizure is one in a series of recent maritime episodes involving Iran.
Shortly after US intelligence raised fears that a foreign tanker had been forced into Iranian waters last weekend, Iran said that it had assisted one vessel suffering a technical glitch.
Since the weekend, US intelligence has been investigating what happened to the vessel, believed to be Panamanian-flagged tanker M/T Riah.
The ship-tracking website Marine Vessel Traffic has not had a current location for that tanker since July 7.
While US intelligence suggested that the tanker was UAE-owned, the United Arab Emirates said on Wednesday that the tanker in question was "neither owned nor operated by the UAE. It does not carry Emirati personnel, and did not emit a distress call," according to state-run WAM.
It is unclear whether the tanker that Iran said on Thursday it had seized is the same vessel as the one that Tehran claimed to have assisted earlier this week.
The IRGC have denied seizing any other tankers, Fars said Thursday.
In another incident last week, armed Iranian boats tried unsuccessfully to impede the passage of a British oil tanker in the Persian Gulf, according to two US officials with direct knowledge of the incident.
In June, tensions between the US and Iran escalated into a military standoff after an American drone was shot down by Iran over the Strait of Hormuz, one of the world's most vital shipping routes.