LAGOS (Reuters) - Rebels in Nigeria said on Friday they had attacked a major oil pipeline in the Niger Delta operated by Royal Dutch Shell, forcing the company to shut in what it described as a small amount of production.

The Movement for the Emancipation of the Niger Delta (MEND) said it attacked the pipeline, which crosses from the Cawthorne Channel to the Bonny terminal at Adamakiri in Rivers state, late on Thursday and planned more such attacks.

"An overfly today confirmed a leak along the Greater Port Harcourt Swamp Pipeline in Eastern operations. The leak appeared to have been caused by explosives," Caroline Wittgen, spokeswoman for Shell in Nigeria, said.

"We have isolated the damaged line and some small quantity of production has been shut in to enable repairs to take place," she said, but declined to give further details on the impact.

MEND announced in September it would recommence attacks on oil installations and the abduction of expatriates following the arrest of one of its leaders, Henry Okah, on arms trafficking charges in Angola. His trial began this month.

"We have decided to step up our attacks ahead of schedule and there will be many more to follow," the group said in a statement on Friday.

Shell -- the company which has been worst hit by Niger Delta violence -- is producing around 400,000 barrels per day below its full capacity from Nigeria because of attacks and kidnappings by MEND since 2006.

The unrest has contributed to a rise in world oil prices, although in recent months, the group has not carried out any attacks which have had a significant impact on oil production.

The rebel group, which demands greater regional control over the Delta's oil wealth, has been riven by factional rivalries. Some of its leaders have expressed willingness to return to peace talks with President Umaru Yar'Adua's government.