BEIJING - Some parts of north China will see the worst smog so far this year from Saturday, the official forecaster said on Thursday.

According to the National Meteorological Center, the air pollution will last through the following Tuesday and be worse than the spell between Dec. 6 and 9, which forced Beijing to issue its highest smog alert for the first time since the emergency response system was created in October 2013.

Visibility in Beijing and some neighboring regions will be reduced to less than one kilometer during the new bout of smog, and the density of PM 2.5 pollution in some of the regions will exceed 500 micrograms per cubic meter, the observatory said. The World Health Organization's recommended maximum is 25 micrograms per cubic meter.

Coal burning and car emissions are some of the major sources of air pollution. In winter, an increase in coal-burning for heating in north China and still weather often exacerbate other forms of pollution and create periods of heavy smog lasting days.