BEIJING -- China's environment minister has urged strict monitoring and crack-down on automobiles with excessive exhaust emissions to improve air quality.

Emissions from mobile sources, such as heavy diesel trucks and old cars, make up a large part of air pollution in the Beijing-Tianjin-Hebei region, said Chen Jining, minister of environmental protection, while inspecting an auto emission management center in Beijing Friday.

The Chinese capital is working to build a city-level environmental monitoring system over automobiles to comprehensively control emissions from mobile sources.

Chen called for accelerating the establishment of a national regulation platform for automobile emissions with a technical support system, as well as a network that monitors high-emission vehicles all the time and from all angles.

He said drivers or owners of vehicles with excessive exhaust emissions should be severely punished in accordance with the law and relevant punishment details will be made public.

Automobile emissions contribute 31.1 percent of Beijing's average PM2.5 density. With a diameter of less than 2.5 micrometers, the particulate matter has been a primary factor behind hazardous smog.