Tensions have flared up in the Niger Delta, the site of much of Nigeria's oil production, since troubled presidential elections last month. Dozens of foreign oil workers have since been kidnapped, at least three pipelines blown up, several supply vessels attacked and oil production curtailed.

Chevron, one of the biggest producers in Nigeria, has begun evacuating hundreds of workers and suspending nonessential offshore activities after four workers were seized from a construction vessel last week. The company called the evacuations "a precautionary measure."

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