BEIJING - China's national observatory on Wednesday renewed a yellow alert for thick fog in some parts of the country.

On Wednesday morning, heavy fog is expected to affect parts of Hebei, Beijing, Tianjin, Liaoning, Sichuan and Chongqing, according to the National Meteorological Center.

Visibility in some affected areas will be reduced to less than 200 meters, the center said.

Drivers are advised to control their speed to ensure safety, while measures should be taken at airports, on expressways and at ferry terminals to ensure traffic safety, according to the center.

China has a three-tier color-coded warning system for thick fog, with red being the most serious, followed by orange and yellow.