"According to international regulations... Iranian forces approached it and using a tugboat brought it into Iranian waters for necessary repairs," Abbas Mousavi said according to an official government twitter account.

The spokesman added that further details would be released later.

Earlier TankerTrackers reported that the Panamanian flagged tanker Riah, used in the Strait of Hormuz "for fuelling other vessels" had crossed into Iranian waters on July 14 "for the 1st time as she slowed down."

According to the online oil shipment tracking service, at that point the tanker's automatic identification system stopped sending signals.

The Riah's last known position was off Qeshm Island in the Strait of Hormuz.

Tensions in the region have soared in recent weeks, with the US calling off air strikes against Iran at the last minute after Tehran downed an American drone, and Washington blaming Tehran for a series of attacks on tanker ships.

On July 4, British Royal Marines helped the Gibraltar authorities detain an Iranian tanker that US officials believe was trying to deliver oil to Syria in violation of separate sets of EU and US sanctions -- a claim denied by Iran.

In a speech on Tuesday Iran's supreme leader Ayatollah Ali Khamenei vowed to retaliate against the British for the ship's seizure.

"The vicious British... have committed piracy and stolen our ship... God willing the Islamic republic will not leave these vicious acts unanswered," he said.