Buildings are seen during a moderately polluted day in Nanjing, Jiangsu Province, China, April 19, 2016. REUTERS/Stringer

BEIJING (Reuters) - Air quality in China’s largest cities continued to improve during the first six months of 2016, the country’s environment ministry said on Sunday.

China’s largest 338 cities enjoyed more clean air days in the first half compared with the same period of 2015, the ministry said on its website. It said 76.7 percent of January-June days had clean air, an increase of four percentage points from a year earlier.

In the capital Beijing, levels of PM 2.5 - dangerous tiny pollutants - fell 17.9 percent from a year earlier, the ministry said.

In Beijing, concentrations of PM 2.5 averaged 64 micrograms per cubic meter in the first half, significantly higher than the official state standard of 35 micrograms, and the World Health Organisation’s guideline of an annual average of no more than 10 micrograms.

The ministry did not provide reasons for the improvement in air quality, but it follows increased measures to crack down on polluters and coal use, as well as efforts to tackle overcapacity in the heavily polluting steel sector. A slowdown in economic growth has also likely contributed.

Of the country’s 10 most polluted cities in the first half, six were in the industrial northern province of Hebei, down from seven at the end of 2015. At the end of last year, the government launched a two-month probe of the province, one of the country’s most polluted regions, and found that firms had engaged in “fraudulent activities” and were flouting orders not to expand industrial capacity.

On July 8, the environment ministry said it had fined several state-owned polluters in May for exceeding emission limits. A subsidiary of oil giant PetroChina Co Ltd in the northeastern city of Dalian was fined 2.9 million yuan ($433,710) it said.