Witnesses said flames from the pipeline were rising several hundred metres into the air.
The pipeline runs close to where the authorities have been trying to plug a leak of hot volcanic mud caused by an oil drilling accident in May.
Police said cracks began to appear in the ground a few hours before the pipeline exploded.
|Volcanic mud has engulfed several |
villages, displacing thousands of people
Rescue officials said that most of those believed killed by the explosion were military and other government personnel involved in trying to stop the mud flow.
Basuki Hadimulyono, head of the national team for mud flow, said: "So far, there are seven bodies that we have been able to get to up until the early hours of Thursday, one of them cannot be identified after being burned beyond recognition."
The mud, flowing out of the ground at a rate of 50,000 cubic metres a day, has swamped several villages and caused a an environmental disaster, displacing more than 10,000 people.