The waters where the US-bound ship was seized, near the Strait of Hormuz, are a chokepoint for at least a third of the world's seaborne oil.

The "Marshall Islands-flagged oil tanker Advantage Sweet was seized by Iran's Islamic Revolutionary Guard Corps Navy while transiting international waters in the Gulf of Oman", the Bahrain-based US Fifth Fleet said in a statement.

"The Iranian government should immediately release the oil tanker," it added, slamming Tehran's "continued harassment of vessels and interference with navigational rights in regional waters".

Iranian state TV announced a "violator ship" had been seized but said it was taken by Iran's navy, rather than the Revolutionary Guard.

"The navy of the Islamic republic of Iran's army seized a violator ship with the flag of the Marshall Islands," the announcement said.

The MarineTraffic tracking website last showed the Advantage Sweet, owned by Advantage Tankers, off the coast of Oman. The crude oil vessel had departed from Kuwait and was en route to Houston, it said.

Iran and the United States have traded barbs in recent years over a spate of incidents in the sensitive waters of the Gulf.

The seizure on Thursday is just the latest incident in the Strait of Hormuz where ships have been mysteriously attacked, drones downed and oil tankers seized.

"In the past two years, Iran has unlawfully seized at least five commercial vessels sailing in the Middle East", the US Navy said.

- Troubled waters -

The seizure comes after the United States, Britain and the European Union toughened sanctions against the Revolutionary Guard Corps on Monday, citing alleged human rights violations by Tehran.

The Western measures add to ones already taken over Tehran's hardline response to protests that rocked Iran since the September death in custody of Mahsa Amini, 22, after her arrest for an alleged breach of strict dress rules for women.

Iran later announced countermeasures, including financial sanctions and entry bans, targeting EU and UK individuals and entities for "imposing and exacerbating cruel sanctions".

Tensions have escalated since 2018 when then US president Donald Trump withdrew the United States from a multinational accord that froze Iran's nuclear programme, and reimposed crippling sanctions on its economy.

In July 2019, the Revolutionary Guards seized the British-flagged oil tanker Stena Impero in the same waterway for allegedly ramming a fishing boat, and released it two months later.

In 2021, Iran released a South Korean oil tanker it had held for months amid a dispute over billions of dollars seized by Seoul.

The waterway is also a notorious smuggling route for weapons to the Iran-backed Huthi rebels in Yemen, who have been fighting a brutal war against Saudi-led coalition that intervened on behalf of the government.

Last month, the British navy said it intercepted a craft packed with arms including anti-tank missiles off the coast of Oman. Tehran dismissed the bust as "fake news".