An oil tanker was boarded in the Gulf of Oman by “unauthorised" people in military uniforms in an apparent seizure on Thursday.
Details remain unclear over the seizure of the ship, the St Nikolas, but suspicion fell on Iran, as the vessel was once involved in a dispute with the US Justice Department that saw a million barrels of Iranian crude oil seized.
It also comes after weeks of attacks by Yemen's Iranian-backed Houthi rebels on shipping in the Red Sea, including their largest barrage ever of drones and missiles launched late on Tuesday.
The chance of possible retaliatory strikes by US-led forces now patrolling the vital waterway rose after a United Nations Security Council vote on Wednesday condemned the Houthis.
Both American and British officials have warned of potential consequences over the attacks on shipping.
The British Maritime Trade Operations, which provides warnings to sailors in the Middle East, said Thursday's apparent seizure began early in the morning, in the waters between Oman and Iran.
The area is used by ships coming in and out of the Strait of Hormuz, the narrow mouth of the Persian Gulf through which a fifth of all oil traded passes.
The group got a report from the ship's security manager of “unknown voices over the phone" alongside the ship's captain.
UK officials said further efforts to contact the ship failed and the men who boarded the vessel wore "black military-style uniforms with black masks".
The private security firm Ambrey said "four to five armed persons" boarded the ship, which it identified as the oil tanker St Nikolas. It said the men covered CCTV cameras as they boarded.
The tanker had been off the city of Basra, Iraq, loading crude oil bound for Aliaga, Turkey, for the Turkish refinery firm Tupras.
Satellite-tracking data analysed by the Associated Press last showed the Marshall Islands-flagged tanker had turned and headed toward the port of Bandar-e Jask in Iran.
The St Nikolas was earlier named the Suez Rajan, associated with the Greek shipping company Empire Navigation.
In a statement, Athens-based Empire Navigation acknowledged losing contact with the vessel, which has a crew of 18 Filipinos and one Greek national, but did not give further details.
