* At least 29 killed in Wednesday's blast
* Survivors suffering severe burns
* Worst industrial accident in India in years (Adds details, updates death toll)
By Malini Menon and Tommy Wilkes
NEW DELHI, Nov 2 (Reuters) - The death toll from a blast at a coal-fired power plant in northern India rose to at least 29 on Thursday, as authorities launched an investigation into the cause of one of the country's worst industrial accidents in years.
More than 20 survivors were battling for their lives with severe burns following Wednesday's blast in a newly-operated unit at the 1,550-megawatt plant run by state-owned NTPC , officials in Uttar Pradesh state said.
More than 80 others suffered injuries in the explosion.
Arvind Kumar, a principal secretary, said some of the severely injured had been taken to a hospital in the state capital Lucknow.
"Blockages in the flue gas pipe in a unit led to the blast. Hot flue gases and steam let out by the blast severely injured several workers," Sanjay Kumar Khatri, the top government official of Rae Bareli district where the plant is located, told Reuters on Thursday.
"A magisterial inquiry has been initiated. This two-member technical team will submit findings within seven days," Khatri said.
In a statement, the National Human Rights Commission said an investigation was needed to ascertain whether negligence or errors had caused the explosion, and asked the state government to submit a detailed report within six weeks.
NTPC is the country's top power producer and accidents have been rare at its facilities.
Senior state police official Anand Kumar said on Wednesday ash had piled up in the furnace beneath the boiler, which led to a build-up of pressure resulting in the explosion.
The federal power ministry and state government have both offered cash compensation to the families of the deceased and to the injured.
The plant in the town of Unchahar supplies electricity to nine states, NTPC's website showed. The company said other facilities would make up for the shortfall and outages were unlikely.
The 500 MW unit had been operating since April and was shut down after the accident. The other five units of the station are operating normally, NTPC said in a statement.
"NTPC has initiated an inquiry into the incident. We are not a company that will take any risk. We have so many units that if power cannot be supplied by one, it can be given by the other. It was a sudden accident," an NTPC official, who did not wish to be named, said. (Editing by Sanjeev Miglani and Michael Perry)