Armed pirates have attacked a ship off the coast of Nigeria, kidnapping four crew members before releasing the vessel, the International Maritime Bureau said on Monday.
It said on its website the attack took place on Sunday about 40 miles off the coast of Nigeria's oil-producing Bayelsa state. It said there were no injuries to other crew members and the ship continued to "a safe port".
The Nigerian Navy did not respond to request for comment.
Piracy and kidnapping in Nigeria's oil-producing Delta and offshore are common, and the oil-rich Gulf of Guinea is second only to the waters off Somalia for the risk of pirate attacks, which drives up insurance costs for oil and shipping firms.