Reuters quoted a Shell spokesman in Nigeria as saying the company was looking into the reports of a spill.
The Movement for the Emancipation of the Niger Delta, Mend, the main delta militant group before a 2009 amnesty, said on Sunday that it destroyed an oil well in the Nembe region of Bayelsa state, where the spill was reported.
But the military said there had not been an attack, according to Reuters.
“The claim by those criminals of blowing up a well head is false,” the news agency quoted military task force spokesman Onyema Nwachukwu as saying.
“Our troops on patrol along Nembe Creek-1 flow station have earlier reported an incident of an oil spill on water in Nembe resulting from the activities of oil thieves.”
Mend claimed responsibility for an attack that killed 12 policemen in Bayelsa last week but police, the military and some ex-militant fighters said it was not carried out by the group.
Tens of thousands of militants from Mend and other groups laid down their arms as part of the 2009 amnesty and security experts believe the media arm of Mend is increasingly disconnected with the bulk of former fighters.
A local community leader was not aware of any oil facilities being attacked.
“There was no explosion, all we see is a leak from a wellhead of a Shell facility,” Reuters quoted Nengi James, chairman of the Nembe community oil committee, as saying.
Oil spills are common in Nigeria, many caused by oil theft carried out by gangs who break into pipelines. Foreign oil companies have also been criticised by rights groups for not maintaining infrastructure or cleaning up spills properly.
Shell estimates that over 150,000 barrels per day of production is lost to oil thieves across the region. The Nembe pipeline is regularly targeted and Shell plans to shut the 150,000 bpd line this month for repairs.