Nigerian officials say gunmen ambushed two vessels in separate incidents Friday off the Niger Delta.
They say at least 10 men in two speedboats attacked an ExxonMobil vessel near Bonny in Rivers state.
They say no one was hurt, but some items were stolen.
Later in the same region, another vessel came under attack by gunmen in three speedboats.
Violence is common in the Niger Delta, where militant groups are demanding the government share more of the region's oil wealth with local residents.
The attacks have cut Nigeria's oil production by about 20 percent from its peak in 2005.
Some information for this report was provided by AP and Reuters.
