Athens-based Dynacom Tankers Management said it had lost communication with the tanker Kerala since Saturday.
"It is suspected that pirates have taken control of the vessel but (this) is not yet confirmed," the company said, adding that it was working with the appropriate authorities to re-establish communication.
"Dynacom is committed to the safety of the crew, environment and the vessel," it said. A company source told AFP that the tanker had been taking on a cargo of oil in the port of the Angolan capital Luanda.
"This is the Atlantic, there has never been a piracy incident in this region," the source said, adding: "But there is no possibility of an accident either."