China's environmental protection chief carried out surprise inspections at two factories on Tuesday night to monitor the implementation of emergency anti-smog measures in the Beijing-Tianjin-Hebei region.

Li Ganjie, the minister of ecology and environment, visited a plant making flux and filler metals for welders and a cement works in Baoding, Hebei province, according to a news release.

Cities across the region have issued air pollution alerts since Monday, triggering measures to restrict the production of smokestack industries.

Hebei has issued an orange alert, the second-highest level of the four-tier warning system used by most authorities. Blue is the lowest, then yellow, and red is the highest.

Beijing - which recently switched to a three-tier system - and Tianjin have issued yellow alerts.

Experts have forecast unfavorable meteorological conditions for dispersing air pollutants this winter, and Li has urged local authorities to adopt a tough stance on environmental violations during periods of heavy smog, the ministry release said.

"All environmental officials should stay in their posts and carry out inspections round the clock to single out any neglected zones for supervision," Li said. 

The release said the ministry had sent notices to Hebei, Shanxi, Shandong, Henan and Shaanxi provinces, as well as Beijing and Tianjin, to request they implement emergency response measures.

Chen Jining, mayor of Beijing and a former environment minister, also visited construction sites and joined inspection teams as they checked diesel trucks at a highway toll gate on Tuesday night. This was after he presided over a video conference to address the emergency response situation, according to media reports.

During the conference, Chen told local authorities to strengthen air pollution control and ensure strict measures are enforced.

This ongoing bout of smog is expected to clear on Friday due to cold air from the north that will arrive in the Beijing-Tianjin-Hebei region at noon on Thursday.