(Adds military comment, details throughout)

By Austin Ekeinde

PORT HARCOURT, Nigeria, Jan 30 (Reuters) - Nigeria's main militant group warned of a "sweeping assault" on the country's oil and gas industry on Friday, saying it was calling off a ceasefire after a military strike on one of its camps.

The Movement for the Emancipation of the Niger Delta (MEND) said the army had carried out an unprovoked attack on the camp of militant leader Ateke Tom and that it would end a four-month ceasefire from 0100 (0000 GMT) on Saturday.

"The oil industry should therefore brace itself for Hurricane Obama, a sweeping assault starting from Rivers state that will change the face of oil and gas exports from Nigeria," the group said in an e-mailed statement.

The joint military taskforce in the delta confirmed there had been clashes, but said its troops had been fired on first.

"Our troops came under fire around Cawthorne Channel in Krakrama (community). The firing was from a distance in the surrounding creeks and our own troops responded," military spokesman Lieutenant-Colonel Sagir Musa said.

Both sides claimed to have killed several of their enemy.

Nigeria is the world's eighth biggest crude oil exporter and also provides around 10 percent of world liquefied natural gas supply, much of it going to Europe and the United States. Previous MEND attacks have pushed world energy prices higher.

Foreign oil firms including ExxonMobil <XOM.N>, Royal Dutch Shell <RDSa.L>, Chevron <CVX.N> and Total <TOTF.PA> all have operations in the Niger Delta, one of the world's largest wetlands and notoriously difficult to patrol.

MEND burst onto the scene in early 2006, knocking out nearly a quarter of Nigeria's oil output within weeks, the start of what it called a fight for more local control of oil resources.

The OPEC member's crude output, then around 2.4 million barrels per day (bpd), has still not fully recovered, languishing below 2 million bpd.

BRITISH HOSTAGES

MEND, which has been holding two British oil workers hostage for more than four months, declared a ceasefire last September but had repeatedly warned it would resume attacks if provoked.

Ateke Tom said the British oil workers were not at his camp.

"I do not have any hostages in my camp. What have I got to do with them? I don't know why the military are attacking me," he told Reuters by telephone.

A spokesman for Ateke Tom's Niger Delta Vigilante, which has links to MEND, said that one of the faction's camps had been attacked by soldiers in six gunboats and several speedboats.

"The battle lasted for almost one hour 30 minutes and we were able to sink one of the double-engined boats with all the occupants," the spokesman, who uses the pseudonym Tamunokuro Ebitari, said.

A private security source said the location of the fighting appeared to be around 20 miles (32 km) from where at least one of the British hostages was believed to be held.

Ebitari blamed Rotimi Amaechi, the governor of Rivers state where the Niger Delta Vigilante is based, for sparking the unrest by using the military to target militant leaders.

"We have been quiet this new year but since Amaechi would not rest until he kills our leader, we are ready for him. Nobody should blame us for whatever steps we take," he said. (For full Reuters Africa coverage and to have your say on the top issues, visit: http://af.reuters.com/ ) (Writing and additional reporting by Nick Tattersall; editing by Mark Trevelyan)

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