Enterprises in suburban Beijing and nearby areas are still illegally discharging airborne pollutants on a large scale, an investigation has found.

This is despite government efforts to fight smog, the inspection by the Ministry of Environmental Protection discovered.

The ministry carried out the inspection with local environmental protection bureaus in Beijing, Hebei and Shanxi provinces, as well as the Inner Mongolia autonomous region, in October.

Twenty-one of the 65 companies and factories checked were found to be carrying out illegal activities, such as excessive emissions of pollutants, or to have poor environmental management, the inspection results released on Monday show.

Dust also worsened air pollution in the region, with no effective dustproof measures taken at construction sites, coal yards and rubbish dumps.

Punishments for the companies are still pending, but nine of them from Hebei have already been fined up to 100,000 yuan ($16,400).

Those in charge of the companies received penalties such as fines, were removed from their posts or dismissed, the ministry said.

Zou Shoumin, director of the ministry's environmental supervision bureau, is in charge of the inspection.

"Under the current laws and regulations, people can be investigated for legal responsibility only when found to have violated relevant laws more than twice a year and to have caused a certain level of damage to the environment.

"We have high expectations for the environmental law that is being amended and hope that harsher penalties, such as fines based on the number of days of pollution, can be included," he said.

Zou said local governments in the Beijing-Tianjin-Hebei area have made great efforts to implement the central government's air pollution prevention action plan in recent months.

For example, Baoding in Hebei, which has ranked among the top 10 most polluted cities in China almost every month, has halted work on all downtown construction sites.

By October, the local government had also closed about 3,000 small, heavily polluting companies that were operating illegally in the electroplating, cement, steel, refining and quarrying industries.

Chen Xintong, an official at the Baoding environmental protection bureau, said, "The government has long been cracking down on these small enterprises, and measures have been intensified this year to fight severe air pollution."

The inspection last month was part of a six-month campaign from October to March focusing on key polluters in the Beijing-Tianjin-Hebei area.

The campaign is aimed at reducing smog that can worsen sharply during the winter heating season.

Zou, from the ministry, said investigations would cover more regions in northern China, where air pollution is severe.

"As the heating season began on Nov 15 in most cities, more problems may occur," he said. "Meanwhile, we will also keep an eye on previous problems and see how they are tackled."

Twelve investigation teams, comprising environmental officials and journalists, will begin a two-week fact-finding tour of 12 cities from Monday.

All problems found during the investigation will be disclosed in early December to inspire greater media and public supervision, ministry spokesman Tao Detian said.