Garuba Yabuku, civil defense spokesman for Emohua, in Rivers State, the heart of Africa's biggest oil industry, said the incident was reported around 4:30 a.m.
Rivers state police said the blast in Rumurkpe community, Emuoha district, was caused by thieves and vandals.
"The victims were scooping crude product when the site caught fire," state police spokeswoman Grace Iringe-Koko said in a statement.
"So far, about 12 persons are believed to have been burnt to death. The identities of the victims are still unknown," she said.
She said some items including five vehicles and one motorcycle were also destroyed.
The police spokeswoman cautioned people against pipeline vandalism and oil thefts which are common in the region and hurt the national oil production and revenues.
The Youths and Environmental AdvocCenter (YEAC), an NGO, said "All those who were at the exact tapping point in the 'pit' got killed while some of those who had loaded the crude and waiting to load including women have been killed by the explosion."
The NGO attributed the disaster to illegal oil refining and thefts.
Oil theft and pipeline sabotage are common in the southern oil production heartland of Nigeria, Africa's biggest crude producer. The methods used to steal oil often result in accidents that cause fires.
Hundreds have been killed in Lagos and the oi-producing south in recent years.
In 2018, a similar disaster killed 60 people.
