The spokeswoman said smoke hampered rescue operations initially and that an investigation would be launched soon. The ship belongs to the country's top energy explorer, Oil and Natural Gas Corp Ltd, she added.
An ONGC official said the ship was undergoing maintenance at state-run Cochin Shipyard on the country's southwest coast.
The blast took place inside the water tank, said M.P. Dinesh, a senior police official in the southern state of Kerala where the shipyard is located.
"The casualties occurred in the smoke that spilled out from the tank, according to preliminary information," he told reporters. "All people trapped in the ship have been evacuated. The situation is under control now."
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