China, in the sixth year of its "war on pollution", is seeking to maintain its focus on curbing smog, but without unnecessarily penalising well-performed producers at a time of slowing economic growth.

Heavy industry includes steelmaking, coal-fired power plants,  cement factories, coke plants, metal casting and chemical production.

"The purpose of this move is to minimize the impact on production at companies during the anti-pollution campaign but meanwhile continue to improve air quality," Ministry of Ecology and Environment (MEE) Spokesman Liu Youbin said.

Some local governments and manufacturers have complained that production cuts have hampered industrial operations and dragged on local economic growth.

Last week, the statistics bureau in Shaanxi province, the latest target in Beijing's anti-pollution campaign, said its industrial growth is under pressure, partly because nearly a quarter of firms in the regions were shut down or forced to cut output.

The environment ministry said it will issue guidance on anti-pollution measures for heavy industry based on production, technology, equipment and transport systems, and will rate manufacturers in three different categories based on their emission levels.

The three categories will face different levels of production curbs, Liu said.

Firms that meet ultra-low emission levels as part of the country's anti-pollution campaign, will be exempt from production restrictions this winter, he said.

China is promoting ultra-low emission standards in coal-fired power and steel industries, and is revising emission standards for industries including pesticide, petroleum and natural gas exploitation, metallurgy and electronic manufacturing.

Beijing ditched blanket winter output curbs for heavy industry last year, allowing local authorities to adopt measures based on regional emission levels.

However, most northern cities failed to meet winter smog targets over the six-months to end-March, which has added to fears that efforts to tackle smog have lost momentum.