Air pollution in Beijing dropped slightly last year, municipal authorities said, although levels of the most dangerous small particulate matter remained more than three times the internationally recommended limit.

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

According to a notice posted Sunday on the website of the Beijing Municipal Environmental Protection Bureau, levels of PM2.5 -- small airborne particles with a diameter small enough to deeply penetrate the lungs -- declined by four percent in 2014 compared with 2013.

The average density for such particles was 85.9 micrograms per cubic metre -- more than triple the World Health Organization's recommended limit of 25 micrograms per cubic metre.

The notoriously smog-clogged capital had 93 days of "excellent air quality" last year, 22 days more than in 2013, the environmental protection bureau said.

There were 45 days of "heavy pollution", down by 13 from 2013, it added.

Public discontent about the environment has grown in China, leading the government to declare a "war on pollution" and vow to cut coal use in some areas.

China's State Council, or Cabinet, said in 2013 that "concentrations of fine particles" in the capital's air would fall by approximately 25 percent from 2012 levels by 2017.

But poor air quality has persisted as officials continue to focus on economic growth, and lax enforcement of environmental regulations remains rife.

In October, thousands of runners battled thick smog during the Beijing Marathon, with photos of mask-clad athletes drawing worldwide attention to the city's dangerously polluted skies. A stage of Tour of Beijing bicycle race was also cut short because of the poor air quality.

In November, authorities enforced a host of anti-pollution measures ahead of the Asia-Pacific Economic Cooperation (APEC) forum, when Chinese President Xi Jinping hosted leaders from the United States, Russia and Japan among others.

The briefly clear skies were popularly dubbed "APEC blue" by online commentators, mocking their temporary nature.

Xi himself made note of the city's pollution problem at a welcome banquet for leaders and spouses, telling the crowd that "these days the first thing I do in the morning is to check the air quality in Beijing".

As smog levels crept back up, however, authorities abruptly ordered one of China's most popular air pollution-reporting apps to remove independent data provided by the US embassy.