Following clashes with security forces guarding facilities in Nigeria's oil-rich Niger Delta region, militants say they have "declared war" on the government.
The Movement for the Emancipation of the Niger Delta (MEND) said it was reacting to violence by the military. The military said it had resisted numerous MEND assaults. Both sides say their opponents experienced heavy losses.
MEND's aggressive campaign for a larger part of the region's oil wealth has reduced Nigeria's oil output by over 20%. Several armed groups of militants are operating in the impoverished delta region, of which MEND are the largest. They regularly kidnap foreign oil workers and sabotage pipelines and oil installations.
The President of Nigeria Umaru Yar'adua is under increasing pressure to crack down on the militants and make the delta more secure for international oil firms.
In an email issued by MEND, the group said it had initiated an "oil war" on the government in response to what it described as unprovoked aerial attacks on its Niger Delta bases.
The group said that oil installations in Rivers state had been targeted by hundreds of boats with heavily-armed fighters on board. MEND stated that an oil platform at Kula, operated by oil giant Chevron, was one of the facilities attacked, and added that 22 Nigerian troops had been killed in the assault.
The Niger Delta region supplies the majority of the Nigerian government's income, yet it remains devastated by corruption and poverty.