The country's top environmental watchdog summoned the mayors of six cities where the air quality deteriorated last winter on Thursday, asking them to draft rectification plans within 20 days.

The six cities, in the Beijing-Tianjin-Hebei province cluster and Fenhe-Weihe Plain area, failed to reach their air pollution control targets between October to March and even saw levels of PM2.5 particulate matter increase, the Ministry of Ecology and Environment said in a media release.

The PM2.5 concentration in Baoding, Hebei, increased by 12.9 percent year-on-year, it said, and there were 10 more days with heavy air pollution than in the same period the previous year.

The ministry said recent inspections it carried out found that the cities, also including Langfang in Hebei and Luoyang and Anyang in Henan province, had relaxed their efforts in the campaign to ensure blue skies.

Baoding only completed about 29 percent of its task to promote the use of less-polluting processed coal as a heating source. The city also failed to collect sufficient funds to subsidize the shift from bulk coal to clean energy for house warming, and 36 percent of villages that had completed the shift switched back to bulk coal for heating, it said.

The PM2.5 density in Luoyang rose 17.3 percent year-on-year during the heating season. The city failed to strictly implement production restrictions to reduce the discharge of pollutants on days with heavy pollution and only managed to rectify 58 of the 65 violations it planned to address, the ministry said.

In a media release issued in late April, the ministry said most of the 39 cities in the Beijing-Tianjin-Hebei region and Fenhe-Weihe Plain area failed to reach their air pollution control targets last winter.

Only four cities, including Beijing, met their targets, while 24 cities saw their air quality worsen. The average PM2.5 density in the Beijing-Tianjin-Hebei region during the heating season was 82 micrograms per cubic meter, up 6.5 percent year-on-year.