WARRI, Nigeria — Shell Nigeria refused Monday to confirm a report by the Niger Delta Avengers militant group that it has bombed the company’s Bonny oil pipeline and again crippled its exports.

Friday night’s bombing breaks a month-long ceasefire between militant groups and the government and comes days after repairs from an earlier attack had allowed Shell’s exports to resume.

Shell spokesman Precious Okolobo said only that “We cannot comment on the reported incident.”

A contractor working at Bonny terminal said production has fallen significantly since Saturday. He spoke on condition of anonymity for fear of losing his contract.

The Niger Delta Avengers said the attack “has brought down oil production activities at the Bonny 48 inches crude oil export line,” which can export 600,000 barrels of oil a day.

The Avengers said the bombing was “only a wake-up call” responding to a clampdown by security forces that it said violated the ceasefire. The military reported arresting at least two Avenger commanders last week.

The ceasefire was a government precondition for negotiations to end attacks, which officials say have halted 40 percent of oil production and thrown Nigeria into recession. Oil accounts for 70 percent of government revenue and 90 percent of exports.

Militants this year have damaged installations of the Dutch-British Shell, the American Chevron and ExxonMobil, the Italian Agip and the French Total. The militants demand that the multinationals leave the Niger Delta, where decades of petroleum production has polluted fishing grounds, agricultural fields and mangrove swamps.