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Pirates have attacked a Turkish cargo ship off the coast of Nigeria kidnapping six of the crew, says the Nigerian navy.

The ship carrying chemicals was believed to be travelling from Gabon to Ivory Coast.

The pirates attacked the ship late at night as it was sailing close to the oil-rich Niger Delta.

Analysts say winding down an amnesty to former Niger Delta militants has resulted in an increase in piracy.

The Nigerian navy say the vessel's captain and the chief engineer were among those kidnapped.

The ship's Turkish owners say none of the crew were injured in the attack but that they have no information on their whereabouts.

Nigerian officials say they are working with Interpol and the country's secret police to secure the crew's release.

Nigeria's coastal areas are increasingly becoming a hotspot for piracy, reports the ´óÏó´«Ã½'s Nigeria correspondent Martin Patience from the commercial capital, Lagos.

Most of Nigeria's oil wealth comes from the Niger Delta, but the area remains underdeveloped.

An amnesty in 2009 for tens of thousands of militants in the region - who receive a monthly stipend from the government - stemmed the level of violence there.

But some of the former militants are believed to have turned to piracy.