Three of the companies failed to cut their production under the emergency response ordered after severe haze hit the region, according to the ministry of environmental protection on Thursday.

"We found that small and private companies had more problems than these three firms and that 15 companies out of the total failed to control air pollution emissions while providing heating services because they lacked facilities," said Zou Shoumin, head of the Environmental Inspection Bureau.

Another 100 small companies in Fushun, Liaoning province, were found to have discharged pollutants directly, worsening air quality, Zou said, adding that such heavy polluters will be severely penalised, the China Daily reported.

The ministry has carried out inspections in recent days in the three northeastern provinces-Heilongjiang, Jilin and Liaoning, which have seen severe smog.

"The inspections will continue, with tougher penalties to deter polluters and improve air quality," Zou added.