Oil giant Royal Dutch Shell said in a statement Wednesday it was forced to halt crude exports from a key terminal in southern Nigeria due to yet another leak on the Trans Forcados Pipeline. The Anglo-Dutch company gave no reason for the leak, but the aging pipeline carries one-fifth of Nigeria’s exports and is subject to regular sabotage by oil thieves and environmental activists, according to the Associated Press.

Shell has blamed repeated thefts and sabotage as the major cause of spills and pollution in Nigeria’s oil-rich south. The Nigerian government estimated crude oil thefts averaging 300,000 barrels per day coupled with losses during repairs has cost the West African nation $12 billion in 2013. Forcados terminal is one of Nigeria’s biggest terminals with capacity to export 400,000 barrels a day.

Shell shut down the oil terminal for weeks in March last year to repair a leak caused by “crude theft point” on an undersea pipeline, a spokesman said at the time. A statement apparently from the Movement for the Emancipation of the Niger Delta, one of the largest militant groups in southern Nigeria, said they had sabotaged the pipeline March 1, 2014, and that scuba divers had caused “further damage to the ongoing repair works” weeks later. Shell denied the additional sabotage, AP reported.

Shell in Nigeria's Niger Delta A picture taken March 22, 2013, shows gas flare at Shell Cawtharine Channel, Nembe Creek in the Niger Delta. Photo: PIUS UTOMI EKPEI/AFP/Getty Images

The group had been largely inactive since 2009, when Nigeria’s then-President Umaru Musa Yar’Adua offered an amnesty deal to thousands of militants, whose vandalism, theft and attacks in six states in the Niger Delta region had cost Nigeria a third of its oil production. The Movement for the Emancipation of the Niger Delta rebels agreed to lay down their arms in return for an unconditional pardon and stipend.

The amnesty program continued under former President Goodluck Jonathan, a southern Christian who in March lost to Muhammadu Buhari in the country's presidential elections. Former Niger Delta militants in January threatened to take control of the region’s oil and unleash violence on the country should Jonathan lose re-election. Buhari, who took office on May 29, said in June that his administration was committed to continuing the amnesty deal, according to Nigerian newspaper Daily Post.