The country's top environmental watchdog reiterated that officials who fail to reach air pollution control targets will be held fully accountable, as it summoned the mayors of six cities on Thursday that experienced deteriorated air quality during the most recent heating season.

The six cities, all located in the Beijing-Tianjin-Hebei province cluster and Fenhe-Weihe Plain area, including Baoding, Langfang in Hebei province, and Luoyang and Anyang in Henan province, failed to reach their air pollution targets from October to March, according to the Ministry of Ecology and Environment.

Liu Bingjiang, head of the ministry's air quality management, said the country experienced marked improvement in its air quality in 2018. In the last heating season, however, air quality in some regions got worse. The Beijing-Tianjin-Hebei province cluster, for example, even saw a sharp increase in the density of PM 2.5 particulate matter.

"Unfavorable meteorological conditions (for air pollutants dispersal) are partly to blame. The deteriorating situation, however, occurred more because local governments failed to make adequate effort and relaxed their control," Liu told mayors.

According to another media release from the ministry in April, the average PM2.5 density in the cluster during the period went up by 6.5 percent year-on-year.

The release also said most of the 39 cities in the cluster and the plain area failed to reach their targets during the season. While 24 of the cities saw air quality get worse, only four, including Beijing, met their targets.

"We consider air pollution control as a war. There is no jesting in war. Promise must be followed with action. And action cannot go without achievement. Those who fail to reach their target will be fully held accountable," he said.

According to the ministry, the average PM 2.5 concentration in Baoding, Hebei, increased by 12.9 percent year-on-year during the period, and the number of days with heavy air pollution in the city went up by 10 days.

The ministry's recent inspections found that Baoding only completed about 29 percent of its task to promote processed coal, which could result in less air pollution.

The city also failed to collect sufficient funds to subsidize the shift from bulk coal to clean energy for heating houses, and inspectors found early this year that 36 percent of villages that had previously shifted to clean energy turned back to bulk coal.

Guo Jianying, mayor of Baoding, said the city government failed to recognize the grim and complicated situation of air pollution control. While it didn't make adequate effort in the work, responsible officials are not held accountable in a timely manner.

He said Baoding will strengthen the management of bulk coal and will set up checkpoints in county-level cities, townships and villages to prevent bulk coal from entering and being burned.

Luoyang saw an even higher increase of PM 2.5 density of 17.3 percent during the season. The city failed to strictly carry out production restriction measures to reduce pollutant discharges during days of heavy pollution and only managed to rectify 58 of 65 violations it had planned to address, the ministry said.

"I feel deeply guilty and ashamed," said Liu Wankang, mayor of Luoyang, adding that the city failed to properly coordinate environmental protection and economic development.

He said 90 percent of officials held accountable for failures in air pollution control in Luoyang are township level officials. Their punishment didn't help in prompting many of the officials to pay enough attention to air pollution control work.