Beijing's environmental authority has issued an orange alert for smog - its second-highest warning level - with heavy air pollution forecast to persist until Wednesday night.

The weather is due mostly to unfavorable conditions for the dispersal of pollutants, the Beijing Environmental Protection Bureau said.

Elementary and secondary schools have been advised to stop outdoor activities, while trucks transporting construction waste, muck, sand and gravel have been banned from the capital's roads while the alert is in place.

Building demolitions and outdoor barbecues are also forbidden.

According to the Ministry of Environmental Protection, the central and southern Beijing-Tianjin-Hebei region is expected to see smog until Thursday, also partly due to unfavorable meteorological conditions.

The concentration of PM2.5 - harmful particulate matter with a diameter of less than 2.5 micrometers - will climb to 200 micrograms per cubic meter or higher in central and southern parts of the region.

At a news conference on Saturday, Huang Runqiu, vice-minister of environmental protection, called for a sober assessment of air quality improvement in the past five years and continuous efforts to fight pollution.

The annual average concentration of PM2.5 in the Beijing-Tianjin-Hebei region has fallen by about 40 percent since 2013. The number of days with heavy air pollution in key cities nationwide has been halved, Huang said.

According to recent analysis, about 30 percent of the improvement was because of favorable meteorological conditions, and the rest was due to efforts made by various parties, he said.

Air pollution is still not completely controlled, with about 70 percent of China's 338 major cities yet to meet the required standards, according to the vice-minister.

"We should be cool-headed... and must not relax in our efforts," said Huang, adding that emissions of pollution remain high in China.

He said the ministry is drafting a three-year plan to further optimize the structure of industry, energy consumption and transportation.

While reducing coal consumption - a major contributor to air pollution in the Beijing-Tianjin-Hebei region, especially during the winter heating season - the government will make efforts to have railways play a bigger role in transportation and decrease the use of heavy trucks, Huang said.

"Without structural changes, complete air pollution control will just be a mere phrase," he added.