Mend has issued a Sunday deadline for oil firm to pull out of the Niger Delta
The main militant group in Nigeria's southern Niger Delta region has destroyed a major oil pipeline.
The Movement for the Emancipation of the Niger Delta (Mend) said it had attacked pipes for a Chevron facility in response to a military offensive.
The 10-day army assault has forced thousands of people from their homes in the remote region where militants say they want a greater share of resources.
Chevron said it had shut down part of its output in response.
It is thought that five flow stations feeding oil to a major plant are now out of action.
"To protect the environment, the incident has led to the shut in of approximately 100,000 BOPD (barrels of oil per day) production from its swamp operations in Delta State," the oil firm said in a statement.
Nigeria is one of the world's major oil exporters but in recent years, militant attacks have cut production by about 20%.
'Cat and mouse'
The offensive is the longest sustained military action the region has seen since the armed conflict between the government and militant groups began in 2006.
Restricted access to the region means journalists have been unable to verify militant claims of military brutality against civilians.
The military say they have rebel leaders on the run and have seized caches of arms and "financial information".
But security sources have told the BBC the Chevron attacks cast doubt on the claimed success of the campaign.
The oil business has until now escaped the current unrest without having to shut down the flow of oil.
Mend said it had made sure of "huge collateral damage" as it carried out the attack.
"We will continue our cat-and-mouse tactics with the Nigerian military until oil export ceases completely," it added.
Nigerian Vice President Goodluck Jonathan, a former governor from the Niger Delta and head of the presidency's attempt to find a resolution to the conflict, told journalists the military offensive in Delta State would end "within days".
But human rights activists in the region believe the military could expand the operation to neighbouring states.
The military has not commented on its plans.
Militant groups in the Niger Delta have flourished amid a lack of governance and rule of law.
They claim to be fighting to help local people benefit from the region's oil wealth but fund their activities with oil theft, extortion and kidnapping.
The Joint Task Force, charged with bringing security to the Delta, has been accused of brutality and corruption.