The Panama-flagged Riah was in Iranian territorial waters near Qeshm Island, which is home to a Revolutionary Guard base, the defence official told the Associated Press (AP) news agency.
“We certainly have suspicions that it was taken,” the official said.
“Could it have broken down or been towed for assistance? That’s a possibility. But the longer there is a period of no contact… it’s going to be a concern.”
The Riah was travelling from the United Arab Emirates (UAE), though a UAE official said it was neither UAE-owned nor operated and had no Emirati personnel on board.
The ship “did not emit a distress call”, the UAE official said.
“We are monitoring the situation with our international partners.”
A spokeswoman for the US Fifth Fleet, based in the Gulf, said they were aware of reports about the tanker.
The mystery surrounding the Riah comes almost a week after a Royal Navy warship was forced to intervene after three Iranian gunboats attempted to block the passage of a British-flagged tanker in the Strait of Hormuz.
HMS Montrose was able to make the vessels move away without a shot being fired.
The Type-23 frigate was providing additional protection to British Heritage, a BP-owned tanker, following a threat by Iran to seize British ships after Royal Marines helped in the detention of an Iranian tanker off the coast of Gibraltar.
Britain has since said it will facilitate the release of the Grace 1 tanker if Iran gives assurances it will not travel to Syria with the oil it is carrying in breach of EU sanctions.
However, Iran’s Supreme Leader Ayatollah Ali Khamenei said on Tuesday his country will retaliate over the seizure of the supertanker carrying 2.1 million barrels of light crude oil, accusing the UK of an act of “piracy”.
“God willing, the Islamic Republic and its committed forces will not leave this evil without a response,” he said.
He did not elaborate.
The heightened threat has prompted the UK to strengthen its naval presence in the Gulf.
The threat level to British-flagged commercial ships in the Gulf has also been raised to its highest level, with vessels urged to steer clear of Iranian waters.
Iran has not yet commented on the Riah tanker.
A number of oil tankers have been targeted in limpet mine attacks over the past two months, blamed by the US on Iran. Tehran denies the charge.
Tensions in the Gulf are part of a standoff between the US and Iran over a landmark nuclear deal between the two countries and other world powers.
President Donald Trump withdrew Washington from the deal last year and re-imposed sanctions on Tehran, but other signatories, including Britain, France and Germany, are trying to hold the accord together.
This is becoming increasingly difficult as Iran has started to exceed limits of uranium production and enrichment in violation of its commitments under the deal.
It has threatened to scrap them altogether unless European countries cushion Iran from the economic impact of the US action.
The Riah, a 58m (190ft) oil tanker, typically made trips from Dubai and Sharjah on the UAE’s west coast before going through the strait and heading to Fujairah on the UAE’s east coast.
However, something happened to the vessel after 11pm on Saturday, according to tracking data.
Giorgos Beleris, from the tracking company Refinitiv, told Sky News it had consistently transmitted signals for the past three months.
“The fact that she went dark makes it very interesting,” he said.
The owner of the tanker is also a mystery.
The ship’s registered owner, Dubai-based Prime Tankers LLC, told AP it had sold the ship to another company called Mouj Al-Bahar. A man who answered a telephone number registered to the firm told AP it did not own any ships.
Author: Deborah Haynes