Five people have been killed by gunmen off Cameroon's coast near the oil-rich Bakassi peninsula.
French oil firm Perenco said a boat was transporting security men to one of its oil platforms when it was attacked.
The Gulf of Guinea is increasingly becoming a target of pirate attacks and kidnappings of oil workers.
Some militants, who demand ransoms for their release, have said they want compensation for Nigerians displaced after Bakassi was given to Cameroon.
"The incident, which is currently being investigated, tragically led to the deaths of five Cameroonian personnel," Perenco said in a statement.
The group - three soldiers, a pilot and a mechanic - were attacked in the early hours of Wednesday.
A series of kidnappings of foreign nationals began after 2008 when the Bakassi region was formally handed over to Cameroon.
The UN's International Court of Justice awarded the territory to Cameroon in 2002, ending a 15-year dispute between Nigeria and Cameroon over ownership of the area.
Bakassi extends into the Gulf of Guinea, an area which may contain up to 10% of the world's oil and gas reserves.