"The rescue operation is under way, with a dozen cranes pressed into clearance work and rescuers are working cautiously to clear debris so that survivors, if any, can be pulled out," Ashok Agrawal, the district administrator, said.

It might take rescuers a further two to three days to remove the debris, he said.

The power plant is being built by the Bharat Aluminum company (Balco), a 51 per cent-owned unit of Sterlite Industries (India) Ltd, owned by Vedanta Plc.

The Indian government holds the remaining 49 per cent.

Vedanta is a London stock exchange-listed company.

In a news briefing shortly after the accident, BK Srivastava, Balco's general manager, said: "A chimney of 275 metres was being constructed, 100 metres were already completed. There was heavy rain and lightning when the incident occurred."

Separately, Raman Singh, Chhattisgarh's chief minister, said a judical probe had been ordered into the accident.