LAGOS, Oct 31 (Reuters) - Nigerian rebels killed a naval officer and wounded at least six others in dawn raid on a navy ship in the oil producing Niger Delta, security sources said on Wednesday.

The raid on NNS Obula on the Pennington River came just hours after the Movement for the Emancipation of the Niger Delta (MEND) freed six foreign workers it seized on Oct. 26 from an offshore oil facility operated by Saipem, a unit of Italy's Eni <ENI.MI>, and SBM Offshore <SBMO.AS>.

The navy gunboat was assigned to guard the EA offshore oilfield operated by Royal Dutch Shell <RDSa.L> in the state of Bayelsa, but was not at the field when the attack occurred, security sources said.

"The vessel was in transit near the Pennington River. One naval personnel was killed and at least six others injured," said one security source who works for a Western oil company in the anarchic delta.

A spokesman for the navy could not immediately give details of the attack.

MEND, which had also claimed responsibility for last week's attack on the EA offshore oilfield, when seven contract workers to Shell were abducted for two days, said the latest raid was to show that the Nigerian military was incapable of protecting oil installations.

"The attack was to again prove to the oil majors and the Nigerian government that the presence of the Nigerian military in the Niger Delta cannot deter an attack nor provide protection to oil facilities when we decide to attack them," MEND said in an email statement to the media.

The group, which says it is fighting for greater control over the region's oil wealth, also claimed responsibility for the Oct. 26 attack on the Mystras vessel, located 53 miles (85 km) offshore and operated by Eni, shutting 50,000 barrels of oil per day output.

Attacks by MEND since early 2006 had already cut Nigerian output by a fifth, contributing to rising world oil prices and forcing thousands of foreigners to leave the Niger Delta.

MEND had stopped its attacks since President Umaru Yar'Adua took office in May promising to address complaints of poverty and neglect in the delta, but threatened to resume its raids after the arrest last month of one of its leaders in Angola.

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