Chevron Corp.'s (NYSE: CVX) onshore activities in Nigeria's Niger Delta have been shut down by a militant attack at its Escravos terminal, a company source said on May 26.

A militant group called the Niger Delta Avengers, which has told oil firms to leave the Delta before the end of May, said late on May 25 it had blown up the facility's mains electricity feed.

"It is a crude line which means all activities in Chevron are grounded," the source told Reuters, without elaborating.

A Twitter account with the group's name said late on May 25: "We Warned #Chevron... but they didn't Listen. @NDAvengers just blow up the Escravos tank farm Main Electricity Feed PipeLine."

A Chevron spokeswoman in the U.S. said on May 26 that it was against policy to comment on the safety and security of personnel and operations.

The Avengers and other militants, who say they are fighting for a greater share of oil profits, an end to pollution and independence for the region, have intensified attacks in recent months, pushing oil output to its lowest in more than 20 years and compounding the problems faced by Africa's largest economy.

Abuja has responded by moving in army reinforcements but British Foreign Minister Philip Hammond said this month President Muhammadu Buhari needed to deal with the root causes of the conflict.

Delta residents, some of whom sympathize with the militants, have long complained of poverty in an area producing oil accounting for 70% of national income.

Buhari has extended an amnesty deal signed with militants in 2009 that stepped up funding for the region. But he has cut funding for the amnesty program and cancelled contracts with former militants to protect the pipelines they used to attack.

Industry sources said that Escravos onshore production accounts for roughly a third of its total output, on average 3.8 million barrels per month in 2014, according to the latest available data from the Nigerian National Petroleum Corp.

Projected exports for the first half of 2016 averaged 167,000 barrels per day (bbl/d).

Escravos production was already down by more than 40,000 bbl/d after a May 5 militant attack on a Chevron offshore facility.

Increased violence over the past few weeks has also made international buyers more reluctant to buy Nigerian crude due to fears of loading delays and cancellations.