Pipeline vandals caused an explosion at a jetty near Nigeria's 210,000-barrels-per-day capacity refinery in the southern oil hub of Port Harcourt, officials said on Monday.
Ralph Ugwu, a spokesman for Nigerian National Petroleum Corp (NNPC), said the blast on Sunday did not damage refinery installations and had no impact on operations.
"The explosion has nothing to do with the NNPC or the refinery but with some people who came to steal crude," Ugwu said.
A local government official said at least seven people died in the blast.
"From what I have been told, there are casualties. Seven persons were said to have lost their lives," Tamuno Williams, chairman of the local government, told journalists.
NNPC and the police said they could not confirm whether there were casualties.
Nigeria is battling worsening oil theft from small-time vandals whose activities often force major pipelines to close and industrial-scale theft from trunklines and loading stations.
London-based think tank Chatham House said last year that oil theft was costing Africa's top producer an estimated $5 billion a year in potential revenue.
Reporting by Chukwudi Akasire and Tife Owolabi