The militants say they are fighting to bring more money to the Niger Delta
Militants in Nigeria's oil-rich Niger Delta region say they have "declared war" on the government after battling security forces guarding facilities.
The Movement for the Emancipation of the Niger Delta (Mend) said it was responding to attacks by the military.
The military said it had repelled several Mend attacks. Both sides say their opponents suffered heavy losses.
Mend's violent campaign for a bigger part of the area's oil wealth has cut Nigeria's oil output by more than 20%.
Mend militants are the largest of several armed groups operating in the impoverished delta region. They frequently kidnap foreign oil workers and sabotage oil installations and pipelines.
Nigerian President Umaru Yar'adua is under pressure to crack down on the militants and make the delta safer for international oil firms.
In an email released by Mend, the group said it had launched an "oil war" on the government in response to what it described as unprovoked aerial attacks on its bases in the Niger Delta.
The group said its heavily-armed fighters had fanned out in hundreds of boats to attack oil installations in Rivers state.
"The operation will continue until the government of Nigeria appreciates that the solution to peace in the Niger Delta is justice, respect and dialogue," the group said.
An oil platform at Kula, operated by oil giant Chevron, was among the facilities targeted, Mend said, adding that 22 Nigerian troops had been killed in the attack.
A Nigerian military spokesman said they had repelled an attack on an oil platform operated by the US company, Chevron. He said the militants had suffered heavy casualties.
The Niger Delta region is the source of most of the Nigerian government's income, yet it remains blighted by poverty and corruption.