Militants say they are fighting for the rights of poor Nigerians
An attack on two oil pipelines in Nigeria's oil-rich Delta region has cut production and raised the price of oil.
Oil giant Shell confirmed it had stopped pumping crude oil through a major pipeline but did not say how much was being held back.
The pipeline attacked by militants carries 130,000 barrels per day, according to Reuters news agency.
Nigeria's oil production had been creeping back up to its Opec quota of 2.2m barrels a day.
A series of violent attacks in recent years had led to a 20% cut in Nigeria's output.
Production, which dropped early this year to 1.7m barrels, rose above 1.9m last week.
The Movement for the Emancipation of the Niger Delta (Mend) claimed responsibility for the attacks.
They attacked a major trunk pipeline in Nembe Creek, Shell confirmed.
The location of the second attack is yet to be confirmed.
Mend told journalists last week they would attack two major pipelines in order to prove they were not being paid protection money to stay away from oil infrastructure.
The head of the Nigerian National Petroleum Corporation last week admitted to paying militants to allow repairs to a pipeline.
The NNPC later said he had been quoted out of context and the money had gone to communities, not to militants.
Earlier, five foreign oil workers seized in Nigeria by militants were freed, security officials told the BBC.
The group, thought to be Russians, were kidnapped on Thursday on the way to a deep offshore oil field.
The boat was hijacked just after leaving its navy escort and taken to a creek in the swampy Niger Delta.
It was the second group of foreigners to be kidnapped and then released in the past four days in the Delta, which has seen a recent surge in violence.
Two people were shot and wounded and eight foreign oil workers kidnapped on Saturday.
The eight, also believed to be Russians, were released hours after they were taken from a tanker on the Bonny River.
In both cases, security officials said no ransom had been paid.
More than 200 foreign oil workers have been kidnapped in the Niger Delta over the past two years, but they are often released after payment of ransom.
The group of five released on Monday worked for the oil services company Saipem.
It is believed the tanker which came under attack belongs to Global Gas and Refining Ltd, a Nigerian subsidiary of US-based Global Energy Inc, which has been stationed along the Bonny river for more than two years.
It is not known who carried out the kidnappings.
Mend ended a unilaterally announced ceasefire after British Prime Minister Gordon Brown said the UK would help train the Nigerian military amid efforts to tackle violence in the Delta.
The militants say they are fighting to ensure that Niger Delta residents see more benefits of the oil wealth.
But some are criminal gangs, making money from ransom payments and stealing oil.