Americans taken from oil ship near Nigeria. Why pirates are moving west.

Two US citizens were kidnapped from an oil supply vessel off the coast of Nigeria. The incident highlights piracy's shift westward from East Africa – tighter security and the promise of new oil have made the continent's western coast an increasing target for pirate attacks.

Pirates kidnapped two US citizens from an oil supply vessel off the coast of Nigeria early Wednesday, officials told Reuters. The State Department and FBI were leading the American response to the incident, which resulted in the kidnapping of the ship's captain and chief engineer.

Piracy has long flourished off Africa's east coast, where lawlessness in Somalia has fueled attacks on large vessels carrying fuels and other goods. But the intervention of international naval forces is forcing pirates west, where a combination of looser security and increased oil production makes the region an appealing target. 

“There has been a worrying trend in the kidnapping of crew from vessels well outside the territorial limits of coastal states in the Gulf of Guinea,” said Pottengal Mukundan, director of the International Maritime Bureau (IMB), an arm of the International Chamber of Commerce, said in a July statement announcing the organization's global piracy report. 

"There continues to be significant under-reporting of attacks," Mr. Mukundan said. "This prevents meaningful response by the authorities and endangers other vessels sailing into the area unaware of the precise nature of the threat.”       

East African attacks are down, following the global downward trend in piracy. Only 75 ships reported attacks off the coast of Somalia and in the Gulf of Aden in 2012, according to IMB. That's down from 237 in 2011, a figure that accounted for a quarter of attacks worldwide. But West African attacks ticked up to 58 in 2012 from 33 in 2010. In the first half of 2013, IMB reported 31 incidents in the Gulf of Guinea.

The attraction is Nigeria, which is Africa's largest oil producer with a capacity of over 3 million barrels of oil per day. In 2011, Nigeria was the fourth-largest foreign oil supplier to the US. Theft and vandalism have hit onshore production hard in recent years, with the country losing $11 billion to crude oil theft and pipeline vandalism between 2009 and 2011. Citing instability in Nigeria, oil major Shell's Profit dropped to $4.6 billion in the second quarter 2013 from $5.7 billion a year earlier. 

Wednesday's attack, along with other recent incidents. suggests attacks are increasingly moving offshore. That trend has troubled west African leaders, who in June called for the establishment of an international naval force in the Gulf of Guinea to cut down on security threats. Naval ships from European Union, China and the US already patrol the waters off Somalia on Africa's east coast.

“The weakness – and sometimes general inadequacy – of maritime policies in Gulf of Guinea states, and the lack of cooperation between them have allowed criminal networks to diversify their activities and gradually extend them away from the coast and out on the high seas, from the Niger delta to Côte d’Ivoire”, Thierry Vircoulon, central Africa project director of International Crisis Group, said in a statement announcing the Brussels-based nonprofit's report on the area last December.