(Adds background, detail) By Nick Tattersall LAGOS, Sept 23 (Reuters) - Nigerian militants accused the military of launching an air assault on their camps in the oil-producing Niger Delta late on Tuesday but said they were maintaining the unilateral ceasefire announced two days ago. The Movement for the Emancipation of the Niger Delta (MEND) said the security forces had attacked camps belonging to two allied militant groups, the Niger Delta Patriotic Force (NPDF) and the Niger Delta Vigilante (NDV). Brigadier-General Mohammed Yusuf, spokesman for Nigeria's defence headquarters, did not confirm or deny a strike but said the joint military task force, comprising ground troops, navy and air force, was a permanent presence in the delta. "We are not at war with anyone. We are there permanently to maintain peace and security against brigands who carry out lawless activities," he told Reuters. "We have a joint task force there, so if the situation demands we use helicopters, we do so," he said. MEND declared a temporary ceasefire on Sunday after a week of the most intense attacks for years on oil platforms, pipelines, flow stations and gas plants in the heartland of Africa's biggest oil and gas industry. The violence cut Nigeria's oil output by at least 150,000 barrels per day and forced Royal Dutch Shell <RDSa.L> to warn it might be unable to meet contractual obligations on crude exports. MEND said it would not call off its ceasefire as a result of the air strike. But it has warned that other armed groups may not take the same stance. "MEND will not play into the hands of the military by retaliating and putting the peace process in jeopardy at this time," the group said in an e-mailed statement. FIGHTING CRIMINALS Security experts say a loose coalition of armed groups operate under the MEND franchise in the anarchic delta, where foreign oil firms including Shell, Chevron <CVX.N>, Total <TOTF.PA> and Agip <ENI.MI> have interests. MEND had warned in Sunday's ceasefire declaration that it would resume its campaign of sabotage against the oil industry if it came under attack again. It has made ceasefire announcements in the past only to break them, blaming military or government provocation. Five decades of oil extraction have left villages in the Niger Delta polluted and impoverished and resentment against the oil industry runs high. Successive governments have pledged to bring development to the vast network of mangrove creeks. The security forces say they are battling not genuine political militants, but criminal gangs which steal and sell oil or kidnap expatriates and wealthy locals for ransom. Analysts say the region will be pacified only if an alternative source of income can be found to the trade in stolen oil, which is worth millions of dollars a day and benefits criminal gangs, corrupt politicians and businessmen alike. (Editing by Tim Pearce)
