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Two oil workers - a Nigerian and a Dutch national - were killed by gunmen who attacked a vessel operated by the South Korean firm Hyundai, near an oil export complex at Bonny Island.

Several others were reported wounded in a gun battle. No group so far has said it carried out the attack.

The country's oil-rich region has seen a series of attacks in recent months, as locals demand a share of the wealth.

The government says instability in the Delta cost some $4.4bn last year.