China is using drones to spy on polluting industries in its attempts to battle the lung-choking smog that frequently engulfs many of its cities.

The deputy minister of environmental protection, Zhai Qing said drones have recently been used in Beijing, Shanxi and Hebei provinces to inspect for pollution. These are some of the worst affected areas of China, with a high number of coal-fired power stations, steel mills and cement plants.

The unmanned aircraft can cover 70 sqkm during a two hour flight. According to the state-run China Daily newspaper the drones have helped the ministry "resolve" over 200 environment-linked cases, and the ministry is considering more drone inspections in other areas.

The ministry has four drones, first introduced in 2012 at a cost of approximately $1.3m (8m renminbi), according to Yang Yipeng, a ministry official.

"You can easily tell from the colour of the smoke – black, purple and brown – that the pollution is over the limit, because if smokestack scrubbers are operating properly, only white smoke is emitted," said Yang in an interview with the Hong Kong based South China Morning Post.

"There were too many chimneys like these, and the drones also captured pictures of flames in the open air ... and that is still only the tip of the iceberg."

While the drones are mainly used to gather evidence about environmental breaches they are also employed to evaluate the performance of local governments in enforcing environmental protection.

"It was very difficult for the law enforcers to collect evidence of violations when they make inspection trips outside Beijing, because locals easily recognise them and polluting factories swiftly suspend production, leaving few traces," said Yang. "The drones, on the contrary, can catch them off guard as few people notice their existence."

An unmanned aerial vehicle flies over Shashi Airport in Jingzhou city, China, during a smog clearing test. Photograph: Hao Tongqian/Corbis

However, environmental campaigners say that while increased monitoring is good, there should be more focus on policy and legal reforms.

"More monitoring and inspections are no doubt the direction to go. In fact, online emission monitoring systems have already been put in place for key enterprises in many provinces in China. Some data have also been gradually disclosed to the public. The key here is to ensure these systems actually function and to expand the coverage to other places," said Li Shuo, climate and energy campaigner with Greenpeace East Asia. There should be a higher cost of non-compliance for polluters, he added.

"When the country puts all its effort to win premier Li Keqiang's recently declared war against pollution, I would rather like to see one that involves less 'wartime machineries' but employs more systematic policy and legal reforms," said Li.

Meanwhile, the environment ministry has also been testing the possibility of using drones to spray chemicals into the air to disperse smog during times of high air pollution.

More than 100 hours of test flights have been carried out using the unmanned aerial vehicle developed by AVIC Aerospace, a subsidiary of the Aviation Industry Corp of China. The chemicals it spays freezes pollutants in the air, preventing them from falling to the ground.