LAGOS, Nigeria - Pirates released a tanker ship and its crew they had seized off the coast of Nigeria's oil-rich southern delta, a piracy watchdog said Saturday.

The International Maritime Bureau said the MT Halifax sailed away from the waters off Nigeria's coast after being seized off the coast of Port Harcourt, the major city of the oil-rich Niger Delta. The bureau had no immediate details about the crew. Private security officials confirmed the boat's hijacking Thursday.

The Halifax, registered in Malta, is managed by Ancora Investment Trust Inc. of Greece. An employee who answered the telephone in the company's Athens office declined to comment Saturday. A profile of the ship on Ancora's company website identified the nationalities of those onboard as Filipino and Indian, with an Italian ship master.

Commodore Kabir Aliyu, a spokesman for Nigeria's navy, said Saturday he had no information about the ship ever being seized.

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

In August, 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.

While pirates in West Africa have been more willing to use violence in their robberies, the latest string of pirate attacks have seen crews typically let go unharmed after the crude oil is stolen from the ships. Analysts believe many of the pirates come from Nigeria, where corrupt law enforcement allows criminality to thrive.

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 the Niger Delta, where thieves tapping pipelines running through swamps steal hundreds of thousands of barrels of oil a day.