A sophisticated attack on a sub-sea pipeline in Nigeria’s Delta might herald a return to the kind of widespread militant violence that crippled the oil industry in Africa’s top producer less than a decade ago.

Attacks on oil facilities have been on the rise in the swamps since President Muhammadu Buhari vowed to shake up a fraud-ridden amnesty program for rebels who stopped blowing up pipelines in 2009 in exchange for cash and generous contracts.

Adding a new dimension, unknown militants – probably using divers – hit a Shell underwater pipeline last month, interrupting oil flows and forcing the company to shut down its 250,000 barrel-a-day Forcados export terminal for weeks.

Nigeria-based diplomats and security experts say the attack showed a level of skill and inside intelligence rarely seen since the 2004-2009 insurgency, which at its height halved Nigeria’s oil output of around 2 millions barrels a day.

“This was an attack that required knowledge of the area and sophisticated equipment,” said a Western security source, asking not to be named. “There were underwater attacks before but none recently. The oil firms are really worried there will be more.”

Militants tend to attack small overland pipelines or flow stations sitting in hard-to-access mosquito-infested creeks.

The underwater attack has cut 15 percent of Nigerian crude output, dealing another blow to Buhari, who is already having to cope with a collapse of oil revenues due to falling prices, a Boko Haram jihadist insurgency in the north and secessionist calls in the southeast.

The strike came a month after authorities issued an arrest warrant for former militant leader Government Ekpemupolo, better known as Tompolo.

Like other ex-rebel leaders Tompolo became a millionaire through the amnesty by winning contracts to protect pipelines he used to blow up in his fight for a greater share of Nigeria’s oil wealth. Others made a fortune with massive oil theft.

Buhari has vowed to end over-priced state contracts and crude theft. But in the Delta many ex-fighters see the hunt for Tompolo as part of a campaign by mainly Muslim northerners, like the president, against the Christian south. The government denies any such motive.

The swampland’s oil provides 70 percent of state income but, like much of the rest of Nigeria, the region has never seen much development. Its roads are pot-holed and villages polluted from oil spills.

“Many angry young men still support the militants because the government is not addressing their grievances,” said Alagoa Morris, an environmental activist in the Delta. “They work for anyone who supports them.”

TROOPS

Authorities have responded by sending troops to protect oil facilities, a move residents say might fuel tensions as villagers will likely see them as invaders sent by Buhari.

“The militarization of the Delta makes it worse,” said Morris. “People see that the government is only interested in the oil production, not their grievances.”

A security official said Tompolo’s men were probably behind the sub-sea attack. Activists say it could have been the work of other ex-rebels frustrated about the region’s poverty.

Tompolo has disappeared from public view since the arrest warrant was issued, and his spokesman, Paul Bebenimibo, could not be reached for comment.

He had links to the Movement for the Emancipation of the Niger Delta (MEND), one of the most powerful militant groups, which attacked oil facilities and kidnapped expatriate workers.

A previously unknown group called the Niger Delta Avengers has claimed responsibility, warning Buhari of more trouble unless he fulfils a long list of demands such as starting development and cleaning up polluted villages.

Reuters was unable to contact the group or verify its statement. In total, ten oil and gas pipelines or other facilities have been attacked in the Delta since the start of the year, security experts say.

DELTA

Tension has been building in the Delta since Buhari defeated president Goodluck Jonathan, a Christian southerner from the region, in presidential polls a year ago.

The government has extended the amnesty but vowed to shift the focus to job training and away from cash payments, which in the past were collected by “generals” for their “boys”.

So far, the attacks have not been as severe as the previous insurgency. But diplomats worry that Delta activists are teaming up with secessionists in the southeast, where Ibgo people who proclaimed an independent state called Biafra sparked a 1967-70 civil war in which more than a million people died.

In the Avengers statement, the group demanded the release of a pro-Biafra leader jailed since October.

On Thursday, groups of former militant leaders denounced the pipeline attacks but also urged Buhari to expand the amnesty to “some of our brothers who are still in the creeks” – still a hideout for militants, pirates and kidnappers.

They also told Buhari the amnesty should be extended to 2019 to give young men time to receive job training.

The government has said it wants to set up vocational centers to train up to 10,000 people annually. But little has happened as the oil price slump has undermined spending plans.

(Additional reporting by Libby George in London, Felix Onuah in Abuja, Anamesere Igboeroteonwu in Onitsha and Tife Owolabi in Yenagoa; Editing by Ed Cropley and Peter Graff)