SHANGHAI - China's environment ministry has summoned the mayors of six northern Chinese cities to a meeting in Beijing to account for their failures to meet winter targets to cut smog, it said in a notice on Thursday.

Average air quality worsened significantly in much of northern China over the winter compliance period from last October to March, prompting concerns the country's 'war on pollution' was losing steam amid an economic slowdown.

The cities named by the Ministry of Ecology and Environment on Thursday were Baoding and Langfang in the steel heartland of Hebei province, as well as Luoyang, Anyang and Puyang in neighbouring Henan and Jinzhong in Shanxi.

"These cities have relaxed their efforts to defend the blue skies, key tasks have not been completed ... and some problems have rebounded," the ministry said.

Inspectors found the city of Langfang in particular had "significantly reduced its focus ... and significantly relaxed its work requirements" when it came to fighting smog, the ministry said.

Anyang, a major steelmaking city, was China's worst performer over the winter, according to Reuters calculations based on official data, with average concentrations of hazardous small particles known as PM2.5 hitting 111 micrograms per cubic metre, 27 percent higher than a year earlier. urn:newsml:reuters.com:*:nL3N217240

The environment ministry said Anyang's decision-making processes were not up to scratch and law enforcement was not sufficiently strict. The city had also failed to make enough progress to "optimise" and restructure its steel sector.

All the cities apart from Jinzhong saw a double-digit rise in PM2.5 during the compliance period, when most of northern China was under pressure to cut emissions by curbing traffic, coal consumption and industrial output.

Jinzhong saw average PM2.5 concentrations rise 4.5 percent over the period, the ministry said. The city had also failed to properly coordinate its anti-smog efforts and not taken action to combat small "scattered" pollution sources.

The ministry said the mayors from the six cities "faced up" to their problems and vowed to win the war on smog in the future.

(Reporting by David Stanway; Editing by Joseph Radford) ((david.stanway@thomsonreuters.com; +86 21 2083 0066; Reuters Messaging: david.stanway.thomsonreuters.com@reuters.net))