An official says 12 sailors are missing after a vessel working on behalf of Chevron Corp. near Nigeria's coast sank.

The Jascon 4, a tug boat, went down Sunday off the coast of Nigeria's Delta state. Sea Trucks Group spokeswoman Corrie van Kessel said Tuesday that divers were searching for missing sailors and an investigation was under way. The Jascon 4 worked for a Sea Trucks subsidiary called West African Ventures.

Working offshore of Nigeria can be dangerous, as sailors face attacks by pirates and militants. Industrial accidents happen as well. A sailor drowned in 2010 after falling overboard from a boat working for French oil firm Total SA.

Chevron, based in San Ramon, California, operates offshore oil sites through a subsidiary in concert with Nigeria's state-run oil company.