Royal Dutch Shell has shut down a tenth of Nigeria’s oil production, after armed militants kidnapped four expatriate oil workers and unknown assailants vandalised a major pipeline on Wednesday.

The incidents followed attacks on pipelines owned by the Nigerian state-owned oil company in December, which disrupted supplies from the world’s eighth-largest oil exporter for several days. Shell officials confirmed that four expatriate contract workers, including an American and a Briton, had been seized on Wednesday by armed militants who boarded a support vessel off Shell’s offshore EA field.