Police said the death toll is likely to increase because at least a dozen other workers suffered severe burns. Numerous injured have suffered severe burns.
A pipe carrying ash from the burning coal exploded late on Wednesday in the newly installed boiler at the power plant in Uttar Pradesh state.
Mr Khatri said the cause of the accident is not yet known.
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.
Police believe that the ash collected in the furnace below the boiler and raised the pressure inside to the point of 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 supplies electricity to nine states in India but the company said outages were unlikely because other facilities would make up for the shortfall.
Three units with a combined capacity of 630 MW are continuing to generate electricity, while the 500 MW unit that was hit by the explosion has been shut down.
More than 100 were injured in the accident at the publicly owned National Thermal Power Corporation's Unchahar plant in Rai Bareli.
Relatives of workers gathered outside the main gate of the power plant desperately seeking information, police officer Suresh Chandra said.
"NTPC has initiated an inquiry into the incident".
Officials said the explosion took place in the boiler which is filled with water in tubes which are heated.