The attack came a day after Mend declared an "oil war" against foreign-owned oil companies working in the region.
The group said it had launched "hurricane Barbarossa" and destroyed flow stations and oil pipelines, killing 22 Nigerian soldiers.
Mend said it was launching the "war" after government troops attacked one of its positions a day earlier with aerial and marine forces.
On Saturday the group warned oil firms to withdraw their workers or face a "hurricane of retaliation".
Violence in the Niger Delta, the heart of Nigeria's oil industry, has halted a fifth of the country's production since 2006.
Royal Dutch Shell, ExxonMobil, Total, Eni and Chevron, are among the numerous oil companies operating in the Niger Delta.