Two cities in northwestern China, Xian and Yinchuan, advised residents to stay indoors, limited heavy industrial production and halted coal processing, warning of heavy pollution over the coming days with thick fog expected.

China's Central Meteorological Observatory also predicted dense haze for the country's north and east from early Thursday to Saturday before a cold wave arrives Saturday afternoon to dissipate the heavy fog.

In Xian, the capital of Shaanxi province, authorities issued a red alert on Wednesday expecting serious pollution until Friday, and enforced mandatory emergency measures to reduce emissions, including asking the public to take public transportation and firms to allow flexible work arrangements.

The city asked companies, institutions, and construction sites that emit pollutants to adjust production schedules and proactively reduce emissions, state television CCTV said.

The capital of Ningxia region, Yinchuan, activated measures from Tuesday afternoon to cut emissions of exhaust pollutants by more than 20% while ensuring enterprises continued normal operating conditions.

Moderate pollution is forecast until Friday in Yinchuan, which was shrouded in grey, photos from China News Service showed, with a chance of severe pollution on Thursday.

The city sought to slow or even stop industrial production, and strengthen pollution controls for key industries such as power stations and petrochemical, chemical, metallurgical and cement plants.

Authorities of both cities stopped most earthwork and demolition operations at construction sites, spray and paint work as well production and transportation of bulk materials using heavy-duty trucks at concrete plants, and in coal processing, stone-cutting and mining activities.

Both cities expanded restrictions on using high-emission vehicles including diesel vehicles, and advised people to avoid idling their vehicles.

The national weather observatory warned of low visibility in dense fog of less than 200 metres in parts of northern and eastern provinces of Hebei, Shandong, Jiangsu, Anhui as well as southwestern Chongqing city, potentially disrupting transport and traffic during peak travel before the new year holiday.

It cautioned people with respiratory and cardiovascular illnesses, as the pollution is mainly due to fine particles that can travel deep into lungs and even the bloodstream.

(Reporting by Liz Lee and Shanghai newsroom; Editing by Sonali Paul)