Fighters riding in about 10 speedboats attacked security personnel guarding an oil-pumping station operated by Royal Dutch Shell PLC in a pre-dawn raid, touching off an hourlong battle, said Lt. Col. Sagir Musa, a spokesman for the military in the southern Niger Delta region.

Musa said the so-called flow station may have been damaged during the battle, but that no government forces were injured. Shell officials couldn’t immediately be reached for comment.

The Movement for the Emancipation of the Niger Delta, the region’s main militant group, had no immediate comment on any involvement. The group has mostly focused on hobbling Nigeria’s oil industry since it emerged nearly three years ago, bombing pipelines in hopes of forcing the federal government to send more revenues to the impoverished oil-producing south.

But a military task force involving marine, land and air forces has stepped up its anti-militant activities in recent weeks, and the militant group said Sunday that two days of relatively rare ground battles with the military meant the region was in a state of war.