The most prominent armed gang in Nigeria's restive oil region, the Movement for the Emancipation of the Niger Delta (MEND), has denied any involvement.

Foreign workers seized
The raid came a day after five foreign oil workers were kidnapped during a similar attack in the same area.
Eleven people, most of them Russians and Filipinos, were originally taken but six were later released along with the vessel.
The five remaining hostages, thought to be Russian, are employed by an engineering company under contract to the French oil giant Total.
They were working on the Akpo deepwater offshore oil field which Total hopes will spring 225,000 barrels of oil per day when it begins production at the turn of the year.
No group has claimed responsibility for either of the latest attacks.
The oil-rich Niger Delta has seen numerous kidnappings targeting  foreign energy firms, claimed by militants demanding a greater share of oil wealth for the region's inhabitants.
On July 11, gunmen seized two Germans working in the restive region.
Relatives and friends of prominent Nigerians have also been targeted in recent weeks by gangs seeking ransom money.
Violence in the southern region has reduced Nigeria's total oil production by a quarter since January 2006.
Nigeria was Africa's largest oil producer until it was overtaken in April by Angola, according to Organisation of Petroleum Exporting Countries (OPEC) figures.