SHIJIAZHUANG, Aug. 6 -- Fifty-six mines have been shut down in north China's Hebei Province, which holds the country's smoggiest cities.

The 56 include 10 illegal mines and 46 others which are within 300 meters of expressways or railways, said Wei Fenghua, an official with the Hebei Land and Resources Department, on Wednesday.

In April, the province launched a campaign to make its mines less polluting and stop them operating near densely populated areas, targeting 632 mines close to railways, expressways and cities.

Most of the 251 mines which failed to meet environmental protection requirements suspended production and underwent treatment while 325 others are due to be treated, said Wei.

Hebei has 14,000 mining sites, including many opencast ones. The mines are a major contributor to air pollution.

Hebei had seven of the 10 most polluted cities in China in the first half of this year, according to the Ministry of Environmental Protection.

Beijing and its surrounding areas, including Tianjin and Hebei, reported the heaviest pollution, though the number of days that met national air standards rose by 3.2 percent compared with the same period last year.

Beijing, Tianjin and Hebei plan to cut PM2.5, a key indicator of air pollution, by 25 percent from 2012 levels by 2017.