The number of "blue sky days" rose by 6.6 percentage points in the first quarter of 2020, said Liu Bingjiang, head of the air pollution office at the Ministry of Ecology and Environment.

"That was a level we didn't even dare imagine," he said, noting the target for the whole 2016-2020 period was 3.3 percentage points.

China has for several months been grappling with a coronavirus outbreak, which has spread around the world and infected more than 4 million people, forcing governments to impose strict lockdowns, severely hurting economic activity.

With millions staying home, concentrations of small lung-damaging floating particles fell by nearly 15% in more than 300 Chinese cities in the first three months of the year, according to official ministry data.

Emissions in the city of Shanghai fell by nearly 20% in the first quarter while Beijing's average emissions levels stood still in the first three months of the year, the data showed.

In Wuhan, where the pandemic originated, monthly averages of particulate matter emissions dropped more than a third.

Normally the country's smoggiest province, Hebei also saw PM2.5 concentrations, a measure of air pollution, fall 15.7% in the first four months of 2020.

The province, however, attributed the drop to its crackdowns on pollution rather than the coronavirus.

New normal 

China has gradually loosened the lockdown since late March in several cities and Liu said the resumption of normal economic activity had not led to a noticeable decline in air quality so far.

He also attributed the improvements in China's air quality to its strenuous anti-pollution efforts over the last few years.

China declared "war" on pollution in 2014 after a spate of politically damaging outbreaks of smog in Beijing and other regions and has been strict to punish those breaking the rules.

The country normally sets targets over the October-March winter heating period, forcing cities to make improvements in air quality compared to the previous year.

But Liu said the steep declines in emissions brought about by the virus would make it too difficult for cities to make further year-on-year cuts next winter, so China would not make the targets a "decisive element" of compliance over the period.

He said China would continue to eliminate small-scale coal use this year and further promote the replacement of coal by gas or electricity in households throughout northern regions, noting that the policies and financial support were now in place.

China is currently working on a new anti-smog plan that would further target ground-level ozone pollution, a rising health risk caused mainly by vehicle exhaust fumes and sunlight.

It will also extend the "blue sky" campaign to other regions, including Jiangsu, Anhui, Shandong and Henan, he said.