Nigeria's security forces have recovered the bodies of 11 of the 12 policemen killed in the oil-rich Niger Delta on Friday, police have said.Some of the bodies had been mutilated and burnt beyond recognition, AFP news agency quoted witnesses as saying.Last week, a militant group said it would it resume attacks after its leader, Henry Okah, was jailed for a bombing campaign in 2010.The oil-rich region is vital to Nigeria's economy.However, many people are poor, fuelling resentment towards the national government and oil companies.At the weekend, the Movement for the Emancipation of the Niger Delta (Mend) said it had ambushed a police boat in the creeks and waterways of Bayelsa state, killing the policemen.Police spokesman Alex Akhigbe said 11 bodies had been recovered, while one was still unaccounted for.The bodies were transported by boat to the regional capital, Yenagoa, while relatives waited at a morgue, Nigeria's Vanguard newspaper reports.At the weekend, police denied the attack was linked to the jailing of Okah.They said it involved a dispute among militants over amnesty payments given by the government.Police boats were escorting an ex-militant to a funeral when one of the boats broke down and became a "soft target" for gunmen, a police spokesman said.Mend had been fighting to gain a greater share of the oil wealth from its part of southern Nigeria, but had been inactive since a 2009 amnesty was put in place.Okah, its leader, was sentenced to 24 years in prison last month for masterminding bomb attacks in the capital, Abuja, in 2010.Nigeria is Africa's largest oil producer.