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The Nigerian military has made six arrests after four soldiers and a policeman were killed in an attack by suspected pirates in the oil-rich Niger Delta on Friday.

Security forces also recovered a cache of arms and ammunition in the raid on their camp, the army said.

Gunmen in four speed boats targeted the military checkpoint in southern Bayelsa state on Friday, authorities said.

Piracy and organised crime linked to oil theft are common in the region.

Soldiers destroyed the camp in neighbouring Rivers state and are now trying to track down others who escaped, according to a statement from the Joint task Force (JTF) in the Niger Delta.

Other items recovered included three speed boats, communication gadgets and generators, it said.

It is not yet clear who was behind the attack.

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

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

But some of the former militants have turned to piracy.