(Adds second invasion)

By Tom Ashby

LAGOS, June 18 (Reuters) - Italian oil company Eni <ENI.MI> said 27 people including 11 soldiers were being held hostage at one of its oilfield stations in Nigeria on Monday after the facility was overrun by militants a day earlier.

The Ogbainbiri flow station in Bayelsa state was one of two oil plants in the Niger Delta invaded by armed men on Sunday, but youths who invaded the second facility, a flow station operated by U.S. oil giant Chevron <CVX.N> at Abiteye in nearby Delta state, had left on Monday.

No one was killed in the invasions, but industry sources said about 80,000 barrels a day of oil production was lost, adding to about 670,000 barrels already shut because of militant attacks in Africa's top oil producer.

Italian oil company Eni said it was working with local authorities to resolve the crisis.

"At the time of the attack there were 24 local workers and 51 soldiers within the premises. 40 soldiers and 8 workers have managed to leave the flow station," the company said in a statement.

That invasion was apparently in response to the killing of eight people by troops guarding Ogbainbiri last week, security sources said. The military said the dead were militants who tried to attack the oilfield, but a militant group said they were mostly unarmed civilians.

In neighbouring Delta state, armed youths invaded Chevron's flow station at Abiteye on Sunday complaining that 150 million naira ($1.2 million) paid to them under a previous compensation scheme had been poorly spent, security sources said.

The youths had left on Monday, but a company spokesman said Chevron would resume 42,000 barrels a day of output there only when it was safe to do so.

SETBACK

The clashes are a setback to a nascent peace initiative by the newly inaugurated President Umaru Yar'Adua and militants who have crippled Africa's largest oil industry over the past 18 months.

A prominent militant leader, Mujahid Dokubo-Asari, was released on bail last week, meeting a key demand of armed groups who have led a campaign of kidnapping of foreign workers and bombings of oil facilities.

Militants have released about 30 hostages since Yar'Adua's inauguration on May 29, and various groups have called a truce to allow dialogue to start. There are at least a dozen other hostages still being held by various other groups in the delta.

Militants complain of neglect and poverty in the delta, which produces all of Nigeria's oil but where most people live without access to electricity, clean water, roads or decent schools. But the line between militancy and crime is blurred and most kidnapping and invasions are by groups seeking money.

Some militant groups want outright independence for the remote region of swamps and mangrove-lined creeks, while others seek more regional control over the oil wealth and compensation for decades of oil spills. (Additional reporting by Randy Fabi in London, Milan bureau)

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