The Youths and Environmental Advocacy Centre said the incident happened at Okrika in Rivers State. Police said the victims died while attempting to steal crude oil.
"Investigation is ongoing to establish the circumstances surrounding the incident and ascertain the facts," said Rivers police spokesperson ASP Blessing Kaborlo Agabe.
The incident, the worst known death toll since at least October 2023, highlights the persistent problem of crude oil theft and pipeline vandalism in the Niger Delta, where illegal tapping has caused deaths, pollution and losses to Nigeria's oil industry.
The environmental group said the incident occurred at Okari Jetty while a vessel was loading and suspected thieves were siphoning a petroleum product under high pressure into boats.
The group said others were missing and bodies had been seen floating in the river.
Reuters was not able to independently confirm what happened and the death toll.
The Nigeria Security and Civil Defence Corps said circulating casualty figures remained unverified and the substance involved required technical confirmation.
It said it was working with security agencies to determine the number of dead, injured and missing, and investigate possible negligence, collusion or criminal facilitation.
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