Raipur, Sep 23 - At least 20 workers were killed and some 50 were trapped under debris when a chimney of an under-construction power plant of Bharat Aluminium Company Ltd (Balco) collapsed in Chhattisgarh's Korba town Wednesday afternoon, police said.

The state government ordered a judicial inquiry into the accident that took place when about 100 workers and engineers were engaged in the construction work at Balco Nagar area, some 250 km north of state capital Raipur.

'It's a massive accident, we have retrieved 20 bodies so far, and many more are trapped under the collapsed chimney's debris. A rescue and relief operation is under way,' District Superintendent of Police Ratanlal Dangi told IANS by telephone.

Balco officials say work was on to construct a 275-metre chimney and construction work was completed for some 100 metres. The chimney structure collapsed amid heavy rain.