BEIJING, Feb. 8 (Xinhua) -- Beijing has installed a camera surveillance system at construction sites for monitoring dust, a major source of PM2.5 in the city, authorities said Thursday.

With the surveillance system, the environmental protection bureau will be able to see and monitor pollutant density around the clock, said an official with the bureau.

Inspectors will warn companies by text message or phone call if problems are found and the construction sites will be under close watch, according to the official.

During a three-month trial companies were warned more than 700 times.

Coal burning, car exhausts, industrial emissions and construction-site dust are among major sources of air pollution in China.

Beijing has tackled smog by closing factories and phasing out coal use. High-emission vehicles and construction site dust will be among major areas for further air pollution control.

The environmental protection bureau on Jan. 18 issued a circular ordering district governments to inspect construction sites and encourage "self-discipline" of construction companies.