The main militant group behind a string of recent attacks in Nigeria's southern oil region sabotaged another pipeline as a workers' strike began hitting ExxonMobil's production in Africa's biggest exporter.

White-collar workers at ExxonMobil Corporation's local joint venture - one of the largest producers in Nigeria, with an output of about 2 million barrels a day in crude - have "commenced a safe and orderly shut-in of production" to push for more pay, the company said.

Officials had no firm details on the amount of oil production lost but said they were operating at "partial production".

The Movement for the Emancipation of the Niger Delta (MEND) said its fighters hit a pipeline late on Thursday in southern Rivers State, bringing to four the number of pipelines the group claims it has blown up in the past week.

The group said in a statement the pipeline belonged to a Royal Dutch Shell joint venture. A Shell spokesman confirmed one of its pipelines had been hit, but had no more details.

MEND says it is fighting to force the government to give more oil industry revenue it controls to its region, which remains deeply poor despite four decades of oil production in the area.

The militants have stepped up activities as one of the group's reputed leaders, Henry Okah, faces trial on terrorism and treason charges. The group emerged two years ago and quickly established itself as the region's most effective militant organisation.

But crime and militancy are intermingled in the region, with gunmen stealing crude oil for resale or robbing banks one day and battling security forces or blowing up oil infrastructure the next.

The southern Niger Delta, where the crude is pumped in Africa's biggest oil industry, is traversed with pipes that carry oil from well heads via transfer stations and on to export terminals. The infrastructure in the vast region of creeks and swamps is virtually unguarded.

Since Okah's arrest, the group has not launched any of the coordinated, military-style armed raids on staffed facilities that originally made it notable.

Shell confirmed three attacks over the past week, and announced it may not be able to meet its obligations to ship about 169,000 barrels per day from Nigeria over the next few weeks. The company, one of the main operators in the country, has yet to report any production outages from the other attacks.

Those attacks helped send crude prices to historic highs on international markets.