The Nigerian military has fired on several barges it says were being used by oil smugglers in the Niger Delta region.
A helicopter gunship carried out the attack Wednesday in Delta state, near the city of Warri.
It was the first known assault by Nigeria's military in the Delta since local militants carried out a series of attacks against the oil industry, including the kidnapping of four foreign oil workers last month.
A local group known as the Movement for the Emancipation of the Niger Delta condemned the helicopter strike, saying it targeted ethnic Ijaw communities.
The group alleges the helicopter took off from an civilian airstrip operated by Royal Dutch Shell. It threatened to shoot down aircraft using the strip if Nigeria's military uses it again.
The militants want greater local control over southern Nigeria's oil wealth.
Some information for this report was provided by AFP and Reuters.