ABUJA, July 17 (Reuters) - A prominent militant group in Nigeria's oil-producing Niger Delta said on Thursday it was not behind an attack on a navy vessel that killed five people and wounded several the previous day.

The Movement for the Emancipation of the Niger Delta (MEND), the main rebel group in the region, the ended a two-week ceasefire on Saturday to protest a British pledge to help Nigeria fight lawlessness in the delta.

"MEND was not involved. The guys are locals," the group's spokesman, who uses the pseudonym Jomo Gbomo, said in an e-mail to Reuters.

About 30 armed men in speedboats attacked a navy vessel on Wednesday that was guarding key oil facilities in southern Rivers state. Three militants, a naval serviceman and a civilian were killed. No group has yet to claim responsibility.

Oil companies and local businesses have been on guard for more attacks since MEND announced last week it would resume its campaign of violence on Africa's biggest oil producer.

Britain and Nigeria agreed on Wednesday to set up a security training force that will include British military experts in an advisory role.

Militants who say they are fighting for greater local control of the Niger Delta's resources have repeatedly targeted the country's oil sector, slashing output by a fifth since early 2006 and helping push world oil prices to record highs.

But the breakdown of law and order in the delta has also allowed criminal gangs to thrive by stealing crude and kidnapping for ransom. (Reporting by Randy Fabi; Editing by Jon Boyle)

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