PORT HARCOURT, Nigeria, Aug 15 (Reuters) - Twelve Nigerian militants and one naval officer were killed in a gunbattle on Friday near a Royal Dutch Shell <RDSa.L> natural gas plant in the oil-producing Niger Delta, military and security sources said.

The militants ambushed two navy gunboats on patrol in Rivers state. Troops returned fire, killing several of the attackers, military spokesman Lieutenant Colonel Sagir Musaitary said.

"On our part there were no casualties just a few injuries," he said.

Two security sources said at least 12 militants and one soldier were killed in the fighting.

The Shell-operated gas plant did not come under fire.

One security source said the gunmen had links to the Movement for the Emancipation of the Niger Delta (MEND).

Militant attacks against Nigeria's oil facilities have become increasingly frequent, shutting down a fifth of oil production since early 2006. (Reporting by Austin Ekeinde; Writing by Randy Fabi; Editing by Angus MacSwan)

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