Pirates have attacked an oil tanker off the coast of Togo, taking control of its bridge and kidnapping 24 sailors before escaping amid an exchange of gunfire with a naval patrol boat, an anti-piracy organization said.

It was not immediately clear if anyone was injured in the attack on the Greek-owned oil tanker, which had been anchored about 19 miles away from Lome, Togo's capital. The pirates took control of the vessel quickly, though an alarm from the ship alerted the Togolese navy, said Noel Choong, an official with the International Maritime Bureau.

The navy boat trailed the tanker and sailors exchanged gunfire with the pirates before the tanker escaped, Choong said.

A naval spokesman could not be immediately reached for comment on Tuesday.

Tuesday's attack is the latest to target West Africa's Gulf of Guinea, which follows the continent's southward curve from Liberia to Gabon. Over the last year, piracy there has escalated from low-level armed robberies to hijackings and cargo thefts.

Violent robberies

Last year, London-based Lloyd's Market Association, an umbrella group of insurers, listed Nigeria, neighbouring Benin and nearby waters in the same risk category as Somalia, where two decades of war and anarchy have allowed piracy to flourish.

Pirates in West Africa have been more willing to use violence in their robberies, as they target the cargo, not the crew for ransom as is the case off Somalia. Analysts say many of the pirates come from Nigeria, where corrupt law enforcement allows criminality to thrive.

The attack on the Greek oil tanker comes about a week after a similar attack on another tanker in the region, Choong said. In that attack the pirates released the crew after stealing the oil onboard, he said.

"Judging the past attacks, they'll take the vessel for several days, ransack it, take the cargo and leave the sailors," Choong said.

Analysts believe the recent hijackings of tanker ships likely is the work of a single, sophisticated criminal gang with knowledge of the oil industry and oil tankers. Those involved in the hijackings may have gotten that experience in Nigeria's southern Niger Delta, where thieves tapping pipelines running through swamps steal hundreds of thousands of barrels of oil a day.