Scientists believe a recent reduction in air pollution across much of China appears to be linked to the county's efforts to control the spread of the new coronavirus.

The outbreak of the virus, which was first detected late last year in the central Hubei province, blocked transport and movement and massively disrupted business activity across the country.

With fewer factories running, cars and lorries on the roads, and planes in the air, the skies have become noticeably clearer.

Levels of nitrogen dioxide, a pollutant which comes primarily from the burning of fossil fuels, were down by as much as 30 percent, according to NASA.

NASA and European Space Agency pollution monitoring satellites issued two images which show a significant change in nitrogen dioxide levels in the air over the country between the start of the year and the end of February.

The first image reveals large pockets of pollution over the big cities, including the capital, Beijing, Shanghai and Hong Kong. The second image shows that most of that pollution has disappeared.

The timing of that improvement in air quality coincides with the moment that government officials in China put Wuhan, the epicentre of the outbreak, into lockdown.

"This is the first time I have seen such a dramatic drop-off over such a wide area for a specific event," said Fei Liu, an air quality researcher at NASA's Goddard Space Flight Center.

While China's carbon emissions are a tiny fraction of its overall annual emissions, it is substantial in a worldwide context, since China is the largest emitter of greenhouse gases.

COVID-19, the disease caused by the virus, has now killed nearly 3,500 people and infected more than 100,000 in over 90 countries and territories. Most cases and deaths have been recorded in China.