The oil pipeline at Omoku in Rivers state had been ruptured before it caught fire early Monday, the sources said. It was not immediately clear if anyone was hurt in the incident.
Firefighters from Agip had put out the blaze.
Meanwhile a fuel tanker exploded Monday morning near Port Harcourt's main oil refinery, military spokesman Major Musa Sagir told AFP. The cause of the blast was unknown.
The two incidents came barely one week after the most prominent militant group in the restive Niger Delta claimed responsibility for a series of attacks.
Last week, the Movement for the Emancipation of the Niger Delta said it had planted an explosive device that set a tanker ship on fire in Port Harcourt.
It also claimed responsibility for an attack on a senior port official in the oil city and vowed further operations against the oil industry and related sectors.
MEND came to prominence early last year with a string of kidnappings of foreign oil workers as well as attacks on oil company property.
The group says that, contrary to criminal gangs operating in the Niger Delta, it is working to improve the lot of the ordinary people of the region.
Instability and violence slashed by a quarter oil output in Nigeria, the world's eighth-largest crude exporter, in 2006 and 2007 to 2.1 million barrels per day, according to the latest estimates.
In 2007, more than 200 foreign workers were taken hostage, often being released after a ransom was paid.