Seven provinces in China witnessed a deterioration in air pollution between 2013 and 2015, though most other regions saw improvements during the same period, news portal thepaper.cn reported Wednesday.

The Chinese Academy of Engineering released Tuesday an interim report on the implementation of the Action against Air Pollution plan on its official website.

The plan, released by the State Council in September 2013, aims to decrease the annual average density of inhalable particles, like PM2.5 and PM10, by 10 percent compared to 2012 levels by the end of 2017 in prefecture-level cities and above across the country.

Based on monitoring data from the Ministry of Environmental Protection, the Chinese Academy of Sciences and the meteorological administration, the report said that the annual average PM2.5 density in the 74 cities which were chosen as pilots for new air quality standards, had declined by 23.6 percent between 2013 and 2015.

The proportion of days with a daily PM2.5 density above 75 micrograms per cubic meter, went from 33.2 percent in 2013 to 20.8 percent in 2015.

However, the report also said that air pollution was still severe across the country.

Seven provinces, including Northeast China's Jilin and Liaoning and Central China's Hubei, had an increase in annual average PM10 densities from 2013 to 2015.

Only 21.6 percent of China's 338 prefecture-level and above cities met the national air quality standards in 2015. Around 23 percent of days were worse than the standard and about 67 percent of days with intense pollution and above occurred in winter, which raised the annual average PM2.5 density.