About 25 Nigerians working for a contractor to Royal Dutch Shell were abducted during a Monday attack on a convoy of boats supplying oilfields, Shell said on Tuesday.
At least three soldiers protecting the convoy were killed when about 70 gunmen in speed boats attacked the barges carrying fuel and other supplies to Shell facilities in the remote Cawthorne Channel in the Rivers state of the Niger Delta.
"We cannot account for the whereabouts of 25 contract staff," a Shell spokesman said.
Monday's attack ended a period of relative quiet in the Niger Delta, which accounts for all oil output from the world's eighth-biggest exporter. A sixth of Nigeria's production capacity has been shut down since February following a wave of militant attacks on oil facilities that month.