Iran has now seized an Iraqi tanker that it had earlier towed following a distress call over the weekend.

State television quoted Iran's Revolutionary Guards as saying it had held the 1,800-dwt Riah (built 1988) over alleged fuel smuggling.

"A foreign vessel smuggling 1m litres of fuel in the Lark Island of the Persian Gulf has been seized," the report said.

The vessel was held on Sunday, it added.

The ship hit the headlines this week after US intelligence sources said it had disappeared from satellite tracking systems after heading into Iranian waters.

Its charterer reported to its technical manager that it had been "hijacked" by Iranian forces.

Iran said the ship has 12 overseas seafarers on board.

"Going dark"

TradeWinds reported on Wednesday that the vessel spent a longer-than-average time trading with its AIS turned off.

Shipping data-analysis company Windward, which specialises in providing security and sanctions surveillance, said it spent 27 days last year “going dark”, as it is known, by turning off its AIS.

Riah has also recorded no port calls in the past year.

When it was transmitting data, it seems to have spent most of its time off Oman and the United Arab Emirates.

But it appeared to be loading cargo by ship-to-ship transfer near Fujairah while “going dark”.

The cargo then seemed to get transhipped to other tankers in Sharjah to be carried to ports in Somalia and Yemen.

Technical manager, Sharjah-based Mouj-al-Bahar, told TradeWinds that it had no knowledge of Riah’s trading patterns.