A Greek-owned oil tanker, identified as the St. Nikolas, has reportedly been boarded by six “unauthorized” men in military uniforms in the Gulf of Oman.

According to reports, the seizure began in the early morning of Thursday in the waters between Oman and Iran in the narrow mouth of the Gulf, in the Strait of Hormuz.

St. Nikolas is a Marshall Islands-flagged tanker associated with the Greek shipping company Empire Navigation. The company reported to the Associated Press that it had lost contact with the vessel and its crew of 18 Filipinos and one Greek national.

The tanker was reportedly loading crude oil off Basra, Iraq and was heading for Aliaga, Turkey to a refinery.

The six men who boarded the ship were wearing black military-style uniforms and black masks, and covered the surveillance cameras of the ship, according to statements made by private security firm Ambrey.