At least eight people died on Saturday in Kom Kom Town in the oil-producing Rivers State, according to an assessment Sunday by the police and a local official.
But locals were "putting the figures at over 70" people killed, the regional coordinator of the National Emergency Management Agency (NEMA), Walson Brandon told AFP on Monday.
It was difficult to confirm the casualties because "the area is inaccessible after heavy rains this morning," he added.
Ogechi Nnamdi, a local resident at the scene said: "I saw about 20 dead bodies. Eleven had been carried away and more are in the swamp yet to be recovered." Promise Chibuzo Nwankwo, a local politician, said the exact numbers of dead were unclear "but I can confirm to you that scores of persons were recorded dead as a result of the explosion."
The explosion occurred on Saturday afternoon while maintenance work was taking place on a pipeline operated by the Pipeline Product Marketing Company, a subsidiary of the state oil company, the Nigerian National Petroleum Corporation.
Leaks had been spotted on the pipeline in the week before the explosion, said Kennedy Azodeh, an official in charge of maintenance operations.
"Locals including vandals stormed the area in boats and canoes to scoop the product", he said.
The company eventually cut off the supply and began maintenance work "accompanied by police officers and members of the Nigeria Security Civil Defence Corps (NSCDC)." "People were asked to leave, but they refused", Azodeh said.
Several oil pipeline accidents have occurred in recent years in Nigeria, Africa's biggest oil producer.
In October 2018, 30 people died in the explosion of an oil pipeline attacked by looters in southeast Nigeria.