Tehran -- Iran's Islamic Revolutionary Guards Corp said it had seized a foreign oil tanker with 12 crew aboard that it accused of smuggling oil in the Persian Gulf.

Iranian state television released video showing small IRGC boats circling the tanker, the MT Riah, a Panama-flagged tanker that went missing four days ago as it transited the Strait of Hormuz close to Iranian waters and which U.S. officials had said they suspected had been seized by Iran.

The seizure comes amid mounting tensions between Iran and the United States in the waters around the Strait of Hormuz and on the same day that President Donald Trump said an American warship had brought down an Iranian drone in the strait. On Thursday, the U.S. State Department condemned Iran taking the tanker and demanded that it release it.

“The United States 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," the department said in a statement.

Hours after the IRGC issued a statement saying it had detained the tanker, President Trump told reporters that the U.S. warship, USS Boxer had destroyed the Iranian drone, saying it had been "threatening the safety of the ship and the ship's crew." Trump said the U.S. ship had taken "defense action" against the drone, saying its flight was "the latest of many provocative and hostile actions by Iran against vessels operating in international waters."

U.S. officials first voiced suspicions the tanker had been seized earlier this week and Iran's foreign ministry on Wednesday initially said the ship had suffered technical problems and had been towed to Iranian waters for repairs. But a day later Iranian authorities now said they had seized the tanker in the act of smuggling a million liters of fuel.

“During the patrolling mission in the Persian Gulf aiming at the discovery and confrontation with organized smuggling on Sunday, 14th of July 2019, the IRGC’s first region navy patrol made the seizure of a foreign vessel by surprise after it made sure the vessel was carrying one million liters smuggled fuel. The seizure was coordinated with the judiciary," the IRGC said in the statement, that was carried by the state Fars news agency. It said the tanker had picked up the oil from small Iranian boats and was sailing towards foreign ships when it was stopped.

The IRGC statement didn't name the tanker but in the videos aired on Iran's state Press TV the name 'Riah' was clearly visible on the ship's stern. The Riah had disappeared on Sunday, after its transponder stopped transmitting, according to ship tracking sites. It's last position was shown close to Iran's Qeshm island in the Strait of Hormuz, that has an IRGC base on it.

The Riah sails under a Panamanian flag but is listed as being operated by a United Arab Emirates' company, Prime Tankers LLC, registered in Dubai, according to the ship tracking site, Fleetmon. The Riah has been widely reported as an Emitrati ship, but this week the UAE's foreign ministry denied that it was owned or operated from there.

Iran's foreign minister, Javad Zarif, on Thursday played down the ship's seizure, telling reporters in New York that is was simply an instance of minor domestic smuggling. "It's not a tanker. It's a small ship carrying a million liters not a million barrels of oil. We do it every other day. These are people who are smuggling our fuel," Zarif said.

After the U.S. allegations earlier in the week that it might have taken the ship in the week Iran's foreign ministry said the vessel had suffered technical problems and had been towed for repairs. Zarif on Thursday did not explain why his ministry had made that claim.

Tensions have been running high in the waters around the strait-- a key waterway through which much of the world's oil is transported-- as Iran has threatened to retaliate over the seizure of an Iranian tanker by Britain close to Gibraltar earlier this month.

Iran and the U.S. have been locked in an increasingly fraught confrontation following the U.S.' withdrawal from the 2015 nuclear deal and as the Trump administration has imposed heavy sanctions in a campaign of what it calls "maximum pressure" against Tehran. Last month, there were two series of attacks on tankers in the Persian Gulf, which the U.S. has blamed on Iran. Iran in the same month also shot down an American drone that it said had entered its airspace. In response, President Donald Trump ordered and then called off a military strike against Iran.

The tensions intensified again after Britain seized the Iranian tanker, the VLCC Grace 1, off the coast of Gibraltar on July 4, prompting threats of retaliation from Iran.

The UK alleged the Iranian tanker was violating European Union sanctions by seeking to deliver oil to Syria. Iran has denied that, saying no Syrian ports are able to receive a vessel the size of the tanker.

Britain said one of its warships was forced to intervene to deter Iranian small military boats that approached a British tanker transiting the strait.