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Iran seized a US-bound oil tanker off Oman on Thursday, saying it had crashed into an Iranian vessel leaving two crew missing - the latest disruptive incident in the crucial but troubled waterway.

The US Navy demanded the immediate release of the Marshall Islands-flagged tanker, saying it was transiting international waters in the Gulf of Oman and slamming Iran's "continued harassment of vessels".

It is one of a spate of incidents since 2018, when then US president Donald Trump pulled out of a nuclear agreement and reimposed crippling sanctions on Iran, sending tensions soaring.

"The Iranian government should immediately release the oil tanker," the Bahrain-based US Fifth Fleet said in a statement.

"Iran's actions are contrary to international law and disruptive to regional security and stability," it added.

The latest flare-up comes only days after Tehran's Western rivals toughened sanctions on Iran's Islamic Revolutionary Guard Corps.

The Fifth Fleet initially blamed the Revolutionary Guards, but then said the capture was carried out by Iran's navy.

It identified the vessel as the oil tanker Advantage Sweet and said it issued a distress call during the seizure.

Iran's navy said the "violator" ship was captured after a collision with an Iranian ship that left two missing and several injured.

"Following the collision of an unknown ship with an Iranian vessel in the waters of the Persian Gulf two of the vessel's crew went missing and several others were injured," the Iranian navy said in a statement.

"The navy of the army, by court order, seized the violator ship, that was fleeing with the flag of the Marshall Islands, and directed it to the coastal waters of the Islamic republic of Iran," it added.

Troubled waters

Iran and the United States have traded barbs over the incidents in the sensitive waters of the Gulf that are a chokepoint for at least a third of the world's seaborne oil.

Thursday's seizure is just the latest incident in the Strait of Hormuz where ships have been mysteriously attacked, drones downed and oil tankers seized since 2018.

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

On Monday, the US, Britain and the European Union toughened sanctions against the Revolutionary Guards, citing alleged human rights violations.

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 the US withdrew from the multinational accord that froze Iran's nuclear programme. Marathon talks to restart the accord have stalled.

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. Last May, Iran also seized two Greek oil tankers.