Pollutants have been reduced by a third in Beijing during this week's temporary restrictions to reduce heavy smog, China's environment ministry said.

Beijing's authorities ordered limits on cars, factories and construction sites during a red alert for smog from Tuesday through to midday on Thursday.

They also told schools to close, though some had reopened by Wednesday.

The Ministry of Environmental Protection said the emergency measures had cut pollutant emissions by 30% from 7am to 5pm local time on Tuesday.

It added that car emissions went down by 32% from Sunday to Tuesday because of the vehicle restrictions that came into effect on Tuesday morning, and because there were fewer cars on the roads on Monday night after authorities announced the restrictions based on odd or even licence plate numbers.

Beijing imposed its first red alert the highest on a four-colour scale that has been in use for two years - following a forecast of high pollution for three consecutive days.

On Thursday, the air quality had improved to a moderate level from previously hazardous levels, according to the US Embassy's air quality index.