China's environmental inspectors named and shamed more cities on Sunday for poor air quality control as the fight against smog continues.

Inspections of 18 cities in north China's Beijing-Tianjin-Hebei (BTH) region and nearby areas used unannounced checks at night and undercover methods, the Ministry of Environmental Protection (MEP) said.

Handan city of Hebei Province continues to illegally operate coal-fired boilers though officials had ordered them closed. After the inspections, the boilers were dismantled and a police investigation opened.

In Cangzhou, also in Hebei, an oil pipe maker, a major source of emissions, was not included in the list of companies to halt production on heavily polluted days.

A cement producer in Beijing used more electricity than usual in December, when it should have suspended production. Two other cement firms were wrongly exempted from production suspensions.

On Friday, the MEP criticized several cities in Hebei, Shanxi and Henan for not doing enough in curbing the use of "scattered coal," coal burned by households or small factories for heating and is much more dirty than that used by thermal plants, which have the equipment to reduce emissions.

China is intensifying efforts to fight pollution and environmental degradation after decades of growth left the country saddled with problems such as smog and contaminated soil.

A total of 720 people were detained and 6,454 held accountable in China for environment-related wrongdoing in 2016, according to earlier official information.

Chinese cities suffered from more days of air pollution in January, with northern areas being the worst affected.

The share of days with good air quality in BTH were a mere 36.2 percent in January, a year-on-year drop of 19.6 percentage points.