Air pollution levels in some of China's smoggiest cities fell by nearly a third in the first quarter of this year, environmental campaign group Greenpeace said on Tuesday.

But pollution levels remain a major public health threat, linked to thousands of early deaths, and the group said they continue to increase in other parts of the country.

In Beijing, levels of PM2.5 -- airborne particulates with a diameter small enough to deeply penetrate the lungs -- fell about 13 percent in the first three months of 2015 compared to the same period a year earlier, Greenpeace said.

The organisation collated data released by China's environmental protection ministry, which makes live figures available but does not publish full historic or comparative statistics.

The main drivers were "the government's strict measures to control air pollution, which have drastically reduced pollution from heavy industry in places like Hebei and Beijing," Zhang Kai, Greenpeace East Asia Climate and Energy Campaigner, wrote in an e-mail to AFP.

Among the 74 cities that have monitored air pollution for more than a year, some saw decreases as much as 48 percent.

Hebei province, which surrounds Beijing and contributes to much of the pollution seen in the capital, saw PM2.5 levels plunge by 31 percent.

"Our expectations for the immediate future is that PM2.5 concentrations in coastal cities will continually improve thanks to measures that the government has put in place to control pollution," he added.

China's cities are often hit by heavy pollution, blamed on coal-burning by power stations and industry, as well as vehicle use.

The issue has become a major source of popular discontent with the Communist Party, leading the government to vow to reduce the proportion of energy derived from fossil fuels.

Kai warned that elsewhere, there were "ever-increasing PM2.5 concentrations in cities in China's central and western provinces, where such measures do not yet exist."

Despite drops in pollution levels in some areas, about 90 percent of 360 cities now being tracked by Greenpeace exceeded government limits on yearly particulate averages.

Shanghai, China's financial hub, saw roughly a 13 percent increase in PM2.5 levels.

China's Premier Li Keqiang said last month that the country was falling short of its people's expectations in battling smog after a popular documentary drew attention to dismal air pollution levels.