Most cities in China saw a slight rise in air pollution last month, the Ministry of Ecology and Environment said in its monthly report on Thursday.

In the 337 cities measured, 76 percent of days in February had good air quality. However, that was 2.3 percentage points less than February last year on average.

The report said the average PM2.5 density nationwide rose by 1.9 percent year-on-year to 55 micrograms per cubic meter, and that northern cities contributed more polluting particles than did cities in the Yangtze River Delta region.

Eleven cities in Henan, Shanxi and Shaanxi provinces performed the worst last month, with an average of seven and a half days having good air quality. Its average PM2.5 readings hit 108 micrograms per cubic meter for February, an increase of 42.1 percent year-on-year.

Twenty-eight cities in the Beijing-Tianjin-Hebei province cluster and nearby areas recorded the same level of PM2.5, up 35 percent compared with the previous February.

Although southern China had better air than the north, it nevertheless was not as good as the previous year. Forty-one cities in the Yangtze River Delta region had an average 20 days of fresh air last month.

The report didn't explain the increasing air pollution. Experts attributed it to weather conditions unfavorable for dispersal, combined with high emissions.

Chai Fahe, a senior researcher at the Chinese Research Academy of Environmental Sciences, said that vehicle emissions and coal combustion were responsible for the increasing pollution in northern China.

"Heavy industries are gathered here and they consume about 40 percent of the country's coal. Also, the heating season led to more coal combustion and pollution," he said.

"In addition, 80 percent of transportation is related to vehicles on highways, which added to emissions."

He said unfavorable meteorological conditions had made things worse and that actions to control emissions should be strengthened.

In June, the State Council released a three-year action plan to curb air pollution by 2020.

It aims to achieve an average of 80 percent of days with good air quality by 2020, with PM 2.5 density set to decrease 18 percent from 2015 levels.