(Updates with presidential aide, Shell spokeswoman)

By Randy Fabi

ABUJA, Sept 14 (Reuters) - Nigeria's most prominent militant group on Sunday declared an "oil war" in the Niger Delta after two days of gunbattles with security forces in the world's eighth largest oil exporter.

The Movement for the Emancipation of the Niger Delta (MEND), responsible for attacks that have cut a fifth of the OPEC member's oil output, repeated its warning for all oil workers to evacuate the delta immediately and expanded its threat to oil vessels.

"MEND has declared an oil war in response to the unprovoked aerial and marine attacks on a MEND position in Rivers state (on Saturday)," the group said in an e-mailed statement.

The deteriorating security situation in the delta, home to Nigeria's oil sector, is considered to be the biggest hindrance on economic growth in Africa's most populous country.

MEND said it had attacked flow stations, a gas plant and oil pipelines, including a Chevron <CVX.N>-operated platform in Kula. It said 22 Nigerian soldiers had been killed, but an army spokesman denied any military casualties.

A Chevron official confirmed one of its oil platforms was attacked, but production had already been shut down due to pipeline problems stemming from a previous attack in late July.

Militants said they also attacked a Shell-operated <RDSa.L> gas plant in Soku and pipelines in Nembe Creek. A Shell spokeswoman said the company was investigating the reports.

Lieutenant Colonel Sagir Musa, spokesman for the military task force in Rivers state, said no oil facilities were affected by two days of heavy fighting.

NIGER DELTA

Security in the Niger Delta worsened dramatically in early 2006 when militants, who say they are fighting for more local control of the impoverished region's oil wealth, started blowing up oil pipelines and kidnapping foreign workers.

"The operation will continue until the government of Nigeria appreciates that the solution to peace in the Niger Delta is justice, respect and dialogue," MEND said.

The heavy fighting began on Saturday in Tombia, in Rivers state, where MEND said at least seven people were killed.

Security sources said Nigeria's army, navy and air force were involved in the clashes, which have spread since Saturday but remain confined to Rivers state.

"At the moment, it seems the military has the upper hand," said a security source with the oil industry, who wished not to be named.

Musa said militants attacked several locations early on Sunday in retaliation for what he said were heavy losses in fighting the previous day. He declined to specify how many were killed.

"There were heavy casualties on the part of the militants. They are losing ground," Musa said. "We are hopeful they will give up the fight very soon."

He said the fighting was provoked by the militants and denied MEND's claims that this was the start of a military offensive in the delta, a vast network of mangrove creeks that is the main source of Nigeria's 2 million barrels per day output.

President Umaru Yar'Adua has been under mounting pressure to bring stability to the delta, one of his many promises upon taking office 16 months ago.

Yar'Adua on Wednesday announced the creation of a new ministry to address the problems of the delta, but militants dismissed it as no more likely to succeed than past efforts.

"Recent attacks on oil installations by the militants are unjustifiable in view that the president is taking concrete actions to solve the Niger Delta problem," said one of Yar'Adua's aides. (Additional reporting by Felix Onuah; Editing by Tume Ahemba and Janet Lawrence)

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