(Adds release of South African hostages, last four paragraphs)

By Austin Ekeinde

PORT HARCOURT, Nigeria, Sept 18 (Reuters) - Nigerian militants said on Thursday they had attacked another oil pipeline in the Niger Delta, hours after the newly appointed chief of defence staff sought to rally troops in the region.

The Movement for the Emancipation of the Niger Delta (MEND), which has declared an "oil war" against the sector and military, said it used explosives to sabotage a Royal Dutch Shell-operated pipeline at the Cawthorne Channel in Rivers state on Thursday.

Shell officials were not immediately available for comment.

Militants in the last six days have bombed pipelines, platforms, gas plants and oilfields, halting up to 150,000 barrels per day of the OPEC member's oil production.

Oil markets found some support from the rise in violence in the world's eighth largest oil exporter, but were mainly focusing on the unravelling global financial crisis. Prices traded lower, below $97 a barrel, on Thursday.

Experts believe the clashes could continue for weeks as the military tries to capture or kill militant leaders and regain control of the region's oil resources. Some private security sources estimate dozens have been killed in the fighting.

Nigeria's Air Chief Marshal Paul Dike, sworn in as chief of defence staff 10 days ago, paid a surprise visit to two army brigades in Rivers state capital Port Harcourt on Wednesday.

"He went to encourage all members of the military task force who are putting their best into the fight against criminality in the region," said a senior army officer.

Fighting between militants and security forces, which began on Saturday, has spread to about 10 villages but has so far been confined to Nigeria's richest state, Rivers.

MEND, responsible for attacks that have cut more than a fifth of Nigeria's oil production, said on Wednesday it would broaden its fight to offshore oilfields and states neighbouring Rivers.

The governor of neighbouring Bayelsa state met Dike on Thursday and asked the military to set up an army headquarters in its capital, Yenagoa, similar to the one in Port Harcourt, the senior officer said.

MEND said on Thursday it released two South African hostages kidnapped more than a week ago in the delta.

The two South Africans, along with two Britons, a Ukrainian and 22 Nigerians, were abducted on Sept. 9 after gunmen seized their vessel in the Niger Delta.

MEND is holding the other 25 oil workers captive as leverage for the release of suspected militant leader Henry Okah, who is in jail for gun-running and treason. (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/ ) (Additional reporting by Felix Onuah in Abuja; Writing by Randy Fabi; editing by Robert Hart)

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