BEIJING (Reuters) - Some northern Chinese cities failed to improve air quality by much last month, hitting the smog-prone region’s overall results in a drive against pollution, the government said as it warned provincial officials to comply with stringent steps to clear the skies.

FILE PHOTO: A man wearing a face mask rides a bicycle on a bridge in front of the financial district of Pudong covered in smog during a polluted day in Shanghai, China November 22, 2017. REUTERS/Aly Song

“Some cities did not improve air quality by much or even experienced some volatility, and in a way, they have dragged down the regional air quality level,” Ministry of Environmental Protection (MEP) spokesperson, Liu Youbin, said at a regular briefing on Thursday.

He did not identify the underperforming cities, but the comment comes amid concerns about the country’s ability to reduce pollution in winter as it battles to avoid a repeat of the near-record levels of choking smog that enveloped key northern areas at the start of the year.

“There is no jesting in war. For those local officials who do not enforce the measures of the campaign effectively and could not improve air quality in time, we will hold them accountable,” Liu added.

Data earlier this month showed only four of 28 northern Chinese cities met their air quality targets in October and air quality in 338 Chinese cities worsened in October, with levels of hazardous breathable particles, known as PM 2.5, up 5.6 percent on the year to an average of 38 micrograms per cubic meter.

Beijing is under huge pressure to meet politically crucial air quality targets and clear the skies of toxic smog that blankets the north of the country as homes turn up the heat which is powered by coal.

Liu said overall air quality in the northern cities was improving compared with September. Average PM 2.5 levels in the region dropped 15.8 percent in October from the month before, he said.

“It shows that our measures are working. As long as we are persistent, and diligently enforce existing measures, regional air quality will definitely improve,” he said.

Under the six-month campaign, 28 northern Chinese cities were ordered to thin traffic and cut industrial output. Thousands of pollution sources including steel mills, coal-fired boilers, cement and ceramic plants, mines and building sites will be shut.

These measures, part of Beijing’s years-long time war on smog, have already roiled commodities market, fuelling worries that the tough inspections are hurting the already slowing economy.