BEIJING, Jan. 4 (Xinhua) -- Beijing extended its orange alert for heavy air pollution again on Wednesday as smog is expected to persist until the weekend, environmental authorities said.

The alert, originally effective from Friday to Sunday last week, was extended for three days on Monday.

The Beijing Municipal Environmental Protection Bureau said air in most of the Beijing-Tianjin-Hebei region remained severely polluted Wednesday night, when the alert was supposed to be lifted. The city's heavy pollution emergency response headquarters has decided to extend the orange alert again till Saturday.

Beijing's air quality is set to improve Sunday night when a strong cold front is forecast to sweep in, but some cities in the Beijing-Tianjin-Hebei region may still see moderate to heavy pollution that day.

From Jan. 9 to 11, air quality in most of the Beijing-Tianjin-Hebei region is expect to be good or slightly polluted, but air quality is forecast to worsen again on Jan. 12.

Air pollution in parts of Shandong, Henan and Shanxi is forecast to disperse between Thursday and Sunday.