Media outlets said two Russians are also part of the 19-person crew.
The ship "disappeared off the radar screens" on Aug. 14, the source said, while regional military officials said the potential search area was between the Gabonese coast and the Sao Tome and Principe archipelago, a hotspot for pirates and ship-hijackers.
Specialist websites list the 121-meter (396-feet) Panamanian-flagged ship, the Pantelena, as a 7,000-ton, 12-year-old dual-purpose oil or chemical tanker.
"The Ministry of Foreign Affairs together with the Marine Transport Agency of the Ministry of Economy and Sustainable Development are working with the shipping company to determine the situation," a statement by the foreign ministry said. A search operation was being conducted with the help of the British maritime authorities, the statement added.
Gabon lies on the southern part of the Gulf of Guinea -- the great bend in the coastline of West African -- where pirates are a notorious problem for shipping.
The Pantelena "turned off its locator beacon," a device that tracks a vessel's position by satellite, a regional military official said.
"The first thing that pirates do when they board a ship is to cut off this beacon."
A crew member aboard a ship sailing between Libreville and Port-Gentil, Gabon's economic hub, told AFP: "We received a distress message over the radio and we alerted the Gabonese navy."
A Gabonese navy official confirmed, "We received an alert... about the Pantelena, but we didn't have enough information to intervene."
In Sao Tome and Principe, which is located about 260 kilometers (160 miles) from Gabon, the commander of the local coastguard, Joao Idalecio, said it had dispatched a patrol vessel with a crew of 30 to search for the tanker.
In February, a Panama-registered tanker, the MT Marine Express with 13,500 tonnes of gasoline was seized with its crew as it was anchored off Benin. The ship and crew were freed several days later.
Last month, the International Maritime Bureau (IMB) said that its specialist piracy reporting center had recorded 107 incidents worldwide in the first six months of 2018.
"All 25 crew kidnappings reported this year have occurred over six incidents in the Gulf of Guinea, highlighting the higher risks in this area," the IMB said.
However, the true number of incidents in the Gulf of Guinea is believed to be "significantly higher," its report added.
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