Nigeria's main militant group has declared a ceasefire, following a week of attacks on oil installations in the country's oil-rich Niger Delta. The Movement for the Emancipation of the Niger Delta (Mend) said it had taken the decision after appeals from tribal leaders in the region. But it warned it would end the truce if attacked by the army again. Mend declared "war" on Nigeria's oil industry last Sunday after a fierce military raid on one of its bases. Groups such as Mend claim to be fighting for greater control over oil wealth in the impoverished Niger Delta, but they are accused of making money from criminal rackets and trade in stolen oil. Cautious welcome "Effective 0100 hours (0000GMT) 21 September, exactly one week after we launched our reprisal, mend will begin a unilateral ceasefire till further notice," the group said in a statement. "We hope that the military has learnt a bitter lesson. The next unprovoked attack will start another oil war," it said. A spokesman for the joint military taskforce, which polices the Niger Delta, cautiously welcomed Mend's move. "We are hoping it will not be another tactical deception which we have already prepared to contend," Lt Col Sagir Musa told Reuters news agency. It is not the first time such a ceasefire has been declared, or this explanation given - and it did not last too long before, the BBC's Alex Last in Lagos says. Two militant factions in the eastern Delta have done the bulk of the fighting, and it may provide room for them to regroup, our correspondent says. He adds that it also may be an attempt to forestall any more major military action. The military says it did not pick this fight, but it has taken a much tougher line against the armed groups operating in the region. Oil production cut In the past week, militants have attacked gas plants, oil installations and pipelines in some of the worst violence for two years. The attacks forced oil giant Shell to declare a force majeure on Saturday - which frees it from contractual obligations - on crude oil shipments from its Niger Delta facilities. Nigeria's oil production has been cut by 20% because of unrest in the region over the past few years. Source: BBC