Shell Nigeria is refusing to confirm a report by Niger Delta militants that they have bombed its Bonny oil pipeline in the south, crippling its exports.
Friday night's bombing breaks a month-long cease-fire between militant groups and the government and comes days after repairs from an earlier attack had allowed exports to resume.
Shell spokesman Precious Okolobo says he cannot comment on the incident.
The Niger Delta Avengers say its attack is "only a wake-up call'' responding to a clampdown by security forces that it says violated the cease-fire. The military has reported arresting at least two Avenger commanders last week.
The cease-fire began as the two sides tried to negotiate an end to attacks, which the government says have halted 40 percent of oil production and causing a recession.