Pirates off Nigeria's coast seized a chemical tanker and kidnapped the vessel's crew, a watchdog group said Tuesday, the latest attack to target shipping in West African waters.

The attack happened Saturday night on a Marshall Islands-flagged vessel about 90 nautical miles south of Lagos, Nigeria's commercial capital, said Cyrus Mody of the International Maritime Bureau. The pirates took control of the vessel and its unknown number of sailors, Mody said.

Mody said he had no other information about the ship or its current status, but said he assumed it remained under pirate control.

"We believe all the crew members are OK at this point in time," Mody said.

Commodore Kabir Aliyu, a spokesman for the Nigerian Navy, declined to immediately comment Tuesday.

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, according to the Denmark-based security firm Risk Intelligence. In August, London-based Lloyd's Market Association, an umbrella group of insurers, listed Nigeria, neighboring Benin and nearby waters in the same risk category as Somalia, where two decades of war and anarchy have allowed piracy to flourish.

West African pirates also have been more willing to use violence -- beating crew members and shooting and stabbing those who get in the way. Analysts believe many of the pirates come from Nigeria, where corrupt law enforcement allows criminality to thrive.

On Sept. 14, armed pirates raided an oil tanker kidnapped 23 sailors off the coast of Benin. The Filipino crew with Spanish, Peruvian and Ukrainian officers ultimately were safely released.

Analysts believe some of the oil tankers targeted carry crude stolen from Nigeria's oil-rich southern delta, where thefts run into the hundred of thousands of barrels of oil a day. Mody said Tuesday that the ship was carrying some sort of chemical or oil cargo at the time it was seized.

"It's definitely the same sort of pattern," Mody said. "It's probably again to do with the stealing of cargo."