Iran's Revolutionary Guard Corps have seized a foreign tanker accused of smuggling oil with a crew of 12, according to Iranian state TV.
The report on Thursday did not identify the impounded vessel, but said it was "smuggling one million litres of fuel" from Iranian smugglers to foreign customers.
The announcement came a day after Iran's foreign ministry said Iranian forces assisted a foreign ship "with technical failure" after it sent out a distress call.
Al Jazeera's Dorsa Jabbari, reporting from Tehran, said the ship impounded was the same one towed by Iranian forces.
"The Iranians say this tanker was initially rescued on Sunday after it sent out distress signals. But once it was towed into Larak island, they say they understood it was smuggling oil and that's when they seized it," she said, adding that the crew were arrested.
"The state TV, quoting the commander of the Revolutionary Guard, said Iran will not stand for this kind of piracy in the Strait of Hormuz."
The developments came amid questions over the whereabouts of a Panamanian-flagged ship travelling through the Strait of Hormuz.
TankerTrackers, an online oil-shipment tracking service, said on Wednesday the Riah had crossed into Iranian waters on Sunday. At that point, the tanker's automatic identification system stopped sending signals.
The "fairly small tanker" was used "for fuelling other vessels" in the Gulf, according to TankerTrackers.
Ranjith Raja, of the data firm Refinitiv, told The Associated Press news agency on Tuesday that the tanker had not switched off its tracking in three months of trips around the United Arab Emirates. "That is a red flag," Raja said.
A UAE official, who requested anonymity, told the AFP news agency on Tuesday that the "tanker in question is neither UAE-owned nor operated".
"[It] does not carry Emirati personnel, and did not emit a distress call," the official said. "We are monitoring the situation with our international partners."
The questions over Riah's status come amid heightened tensions between Tehran and Washington over the former's nuclear programme and a standoff between Iran and the United Kingdom over the British Royal Marines' seizure of an Iranian oil tanker earlier in July.
The vessel was held on the suspicion it was heading to Syria in violation of European Union sanctions.
Tehran has called on the UK to release the vessel immediately, with Iran's Supreme Leader Ali Hosseini Khamenei saying his country will retaliate over the seizure of the supertanker carrying 2.1 million barrels of light crude oil.
Calling the incident an act of "piracy" in a televised speech on Tuesday, Khamenei said: "God willing, the Islamic republic and its committed forces will not leave this evil without a response." He did not elaborate.
The British foreign secretary, Jeremy Hunt, said on Saturday the UK would facilitate the release of the ship if Iran could guarantee the vessel would not breach EU sanctions on oil shipments to Syria.
Separately, the US has also blamed Iran for a spate of suspected attacks in May and June on commercial vessels near the Strait of Hormuz, a vital sea lane through which more than 20% of all crude oil passes. Tehran denies the allegations.
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