BEIJING (REUTERS) - China's northern Hebei province, home to the country's top steelmaking cities, has issued an orange smog alert from Friday (Dec 13) as adverse weather conditions are set to increase pollution, state-backed Hebei News reported on Wednesday.

A total of 11 cities, including provincial capital Shijiazhuang and steel hub Tangshan, will impose the orange alert from 12 noon on Friday, the newspaper said, adding it was not yet decided when it would be lifted.

Orange alerts, the second-highest after red in Hebei's three-tier pollution warning system, require companies to take action to cut emissions and in some cases restrict output, although the province has a "positive list" exempting thousands of firms from production cuts.

The alert comes as winds drop and accumulations of breathable particles known as PM2.5 are set to increase in central, southern and eastern parts of Hebei, which surrounds the capital Beijing, the Hebei News said.

Further south, PM2.5 concentrations in the Yangtze River Delta, including Shanghai, were among the highest in the country on Wednesday.

China's Environment Ministry warned last week that "unfavourable weather" from December to February had increased the likelihood of smog build-ups over the period.