LAGOS (Reuters) - A Nigerian militant group on Wednesday denied issuing a threat of attacks on fuel depots and transporters and said it was investigating how the warning had been sent from its email address. The threat on Tuesday of a "ferocious attack" led the military to urge companies in the downstream oil sector -- which includes refiners and fuel distribution firms -- to increase surveillance of their infrastructure and fleets. "The Movement for the Emancipation of the Niger Delta (MEND) denies in its totality the recent warning of an attack on the downstream oil sector," the group said in an email sent from the same address and using the same pseudonym, Jomo Gbomo. "We will be investigating the situation and will take appropriate actions to prevent this from recurring in the future," it said, adding it would only issue statements claiming attacks once they had been carried out. MEND was for years the main militant group in the Niger Delta, blowing up pipelines in Africa's biggest oil and gas industry. But it has splintered since an amnesty in 2009 and it is not clear which faction now controls the email address.