Pirates have kidnapped 19 crew members from a crude oil tanker off Nigeria in an area where acts of piracy are on the rise, an official with the ship's operator says.

The loaded vessel, the Nave Constellation, was attacked 77 nautical miles off Bonny Island on Tuesday and 18 Indians and one Turk from the crew were seized, the official said.

Seven other crew members remain on board the vessel. Neither the vessel nor the cargo were damaged, the official said.

The shipping industry has warned in recent months about the increasing dangers faced by seafarers sailing through the Gulf of Guinean, particularly around Nigeria, including kidnappings by pirate gangs.

The International Maritime Bureau said in October the Gulf of Guinea accounted for more than four fifths of crew kidnappings globally.

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Maritime security officials say that over the past year there has been a growing shift by pirate gangs in the Gulf of Guinea and especially Nigeria towards kidnapping crews rather than stealing cargoes, aiming to extract ransom from ship owners for crews being held and subjected to violence.

We are doing everything necessary to make sure that the 19 crew members will return safe

The Hong Kong-flagged supertanker, capable of carrying up to 2 million barrels of oil, is operated by Greek shipping company Navios Tankers Management.

"We are doing everything necessary to make sure that the 19 crew members will return safe," the official at Navios said, adding that their families have been informed.

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An Indian government source said its mission in Nigeria had taken up the matter of the kidnappings with the Nigerian government and security agencies.

The vessel was carrying crude for Indian refiner Hindustan Petroleum Corp, a source at the company said.

The vessel was chartered by French oil company Total to deliver Bonny Light oil at Vizag in southern India, the source said.

Nigerian Navy commander Captain Kolawole Oguntuga said the vessel, which loaded Shell Bonny Light, was hijacked soon after leaving the terminal. The ship did not request a navy escort to provide them with adequate protection, he said.