BEIJING (REUTERS) - China faces difficulties in meeting its smog-fighting target for 2017, its environmental protection minister said during a visit to four heavily industrialised provinces in northern China, where the country's air pollution problem is especially acute.

China has pledged to cut average concentrations of hazardous airborne particles known as PM2.5 by more than 15 per cent in the coming winter months from a year earlier in 28 northern cities.

"Currently, the air pollution control work is challenging and complex," Mr Li Ganjie, Minister of Environmental Protection, said during a tour earlier this week to Hebei, Shanxi, Shandong, and Henan provinces, according to a statement posted on the ministry's website on Wednesday (Oct 4).

He said air quality has fluctuated, with some areas seeing a significant slowdown in improvement, and even a worsening.

"The completion of the annual targets for air quality control faces huge difficulties," Mr Li said.

In the capital region, which includes Beijing, surrounding Hebei province and the city of Tianjin, air quality worsened in the area's 13 cities during August from a year earlier, with a 5.4 per cent increase in concentrations of PM2.5, according to the most recent ministry data.

Air quality in China's 338 largest cities on average deteriorated in the first six months of 2017, ministry data showed, with 74.1 per cent of all days during the period experiencing clean air, down 2.6 percentage points from a year earlier.

Mr Li urged local governments to step up enforcement of environmental laws and regulations.

China has promised to close twice as many factories and enforce bigger emission cuts in coming months in a bid to avoid a repeat of the near-record levels of choking smog that enveloped key northern regions at the start of the year.

Mr Li was named as China's new environment minister in June, promising a "protracted battle" to clean up the nation's notoriously polluted air, water and soil.