Nearly 300 Chinese cities failed to meet national standards for air quality last year, Greenpeace said Wednesday, despite marginal improvements in some of the worst-hit areas.

China's cities are often hit by heavy pollution, blamed on coal-burning by power stations, heavy industry and vehicle use, and it has become a major source of discontent with the ruling Communist Party.

The average level of PM2.5 particulates -- small enough to deeply penetrate the lungs -- in the 366 cities monitored was more than five times the maximum recommended by the World Health Organization (WHO), government data compiled by Greenpeace showed.

Of those monitored -- which include all of the nation's major cities -- a total of 293, or 80 percent, were higher than China's own looser national standards.

China allows for a yearly average of 35 micrograms per cubic metre, versus the annual WHO standard of 10 micrograms per cubic metre. None of the cities in the survey met WHO standards.

The capital Beijing was the 27th most polluted city with an average of 80.4 micrograms per cubic metre, a mere 3.3 percent decrease. Beijing had 26 days of "heavy air pollution" in the final three months of last year, according to Greenpeace.

South of the capital, Baoding in the industrial heartland of Hebei province was China's second most polluted city in 2015, averaging 107 micrograms per cubic metre.

"The principal reason for this higher frequency of smog in Beijing and surrounding areas this winter was wind and humidity conditions," Greenpeace said in a statement accompanying the figures.

"Though weather conditions help smog develop, the origin of the pollution remains heavy coal burning across northern China."

Despite years of chronic dirty air, Beijing only issued its first red pollution alert -- reserved for when authorities forecast an air-quality index of above 300 for at least three consecutive days -- in December.

The Greenpeace statistics were based on official data from China's ministry of environmental protection. It makes current levels available online but does not publicly release historical data or averages.

- Falling short -

Air pollution levels in Shanghai, the country's financial centre, rose 3.1 percent in 2015 to 53.9 micrograms per cubic metre.

The most polluted city, with a yearly average of 119.1 micrograms per cubic metre, was Kashgar, near the Pakistani border in the far western region of Xinjiang, which is often hit by dust and sand storms.

Authorities expanded the list of cities monitored to 367 in 2015 from 189. Greenpeace excluded one city in its rankings because of data inconsistencies.

Public discontent about the environment has grown in China, leading the government to declare a "war on pollution" and vow to reduce the proportion of energy derived from fossil fuels. But it has shied away from pledging to cut total national coal use.

Premier Li Keqiang has admitted the government is not doing enough to tackle the massive public health problem.

"The Chinese government is determined to tackle smog and pollution," Li said at the annual meeting of the country's rubber-stamp parliament in March. "The progress we have made still falls short of the expectations of our people."

Despite the frank comments, China's leadership is still highly sensitive to grassroots efforts to highlight the problem.

A scathing independent video on China's persistent air pollution, "Under the Dome," racked up hundreds of millions of views last year before it was taken offline as authorities clamped down.