Mend launched what it called Hurricane Barbarossa last Sunday
Nigeria's main militant group says it has blown up a major oil pipeline owned by Shell in the oil-rich Niger Delta.
The Movement for the Emancipation of the Niger Delta (Mend) said it had used explosives to attack a "major pipeline" in the Asari Toru region.
Mend declared "war" on Nigeria's oil industry last Sunday after a fierce military raid on one of its bases.
If confirmed, the latest attack would be the sixth since Mend launched what it has called Hurricane Barbarossa.
In the past week, militants have attacked gas plants, oil installations and pipelines in some of the worst violence for two years.
Shell, which has only confirmed the first two of the claimed attacks, said it was investigating the latest claim.
The oil giant also declared a force majeure - which frees it from contractual obligations - on crude oil shipments from its Niger Delta facilities.
Mend vowed to "continue to nibble every day at the oil infrastructure in Nigeria until the oil exports reach zero".
Delta militants have carried out a spate of attacks this week
"The military and the government of Nigeria whose unprovoked attack on our position prompted this oil war are no match for a guerrilla insurgency of this kind," it said in a statement.
Groups such as Mend claim to be fighting for greater control over oil wealth in the impoverished Niger Delta, but they are accused of making money from criminal rackets and trade in stolen oil.
Critics say the militants are simply criminal gangs out to extort money from oil companies.
Nigeria's oil production has been cut by 20% because of unrest in the region over the past few years.