BEIJING ?-- Beijing will extend the orange alert for heavy air pollution for another three days as heavy smog will continue in the city, environmental authorities said Monday.

The alert, which was originally effective from Friday to Sunday, will not be lifted until Wednesday midnight, according to a statement issued by the Beijing Municipal Environmental Protection Bureau.

Air quality will improve Monday because of weak cold air, but will return to the medium or heavy air pollution level on Tuesday and Wednesday, the statement said.

Under the emergency plan, heavy polluting gasoline-fuelled vehicles and trucks carrying construction garbage are banned from roads. Some manufacturing firms are required to cut production.

Beijing has a four-tier alert system for pollution, with red the highest, followed by orange, yellow and blue. The orange alert means the air quality index is forecast to exceed 200 for three consecutive days.