The Nigerian military says an army helicopter has attacked barges used in oil-smuggling in the Niger Delta.
The militant group the Movement for the Emancipation of the Niger Delta however condemned the raid and accused the military of targeting civilians.
The attack came as UK Foreign Secretary Jack Straw paid a visit to the oil city Port Harcourt, in the Niger Delta.
There has been a recent spate of violent incidents involving oil industry facilities in the Niger Delta.
Change of tactics
A spokesman for the military task force in the Niger Delta, Major Said Hammed, said one of its helicopters attacked eight barges being used to smuggle oil after spotting them on a routine flight near the village of Okerenkoko, about 30km west of the town of Warri.
Maj Hammed said it was not clear if there were any casualties.
The Movement for the Emancipation of the Niger Delta (Mend) released a statement immediately after the raid saying the helicopter had fired rockets and machine-guns at targets on land and accused the military of targeting civilians.
The Nigerian military does not normally attack barges used in smuggling. Maj Hammed said such a raid was a new directive.
It is the first major military operation in the Niger Delta since Mend carried out a string of attacks on the oil industry, including the kidnapping of four foreign oil workers who were held hostage for almost three weeks.
The militants have threatened to continue attacks unless local communities are given greater control of the oil wealth produced on their land.
The military operation comes as Mr Straw paid a quick visit to Port Harcourt in the eastern Niger Delta at the end of a three-day visit to Nigeria.