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

From Wednesday morning to noon, thick fog is expected to affect parts of Shandong, Anhui, Jiangsu, Zhejiang, Jiangxi, Hunan and Guizhou, according to the National Meteorological Center (NMC).

In some regions of Anhui, Hunan and Guizhou, visibility may be reduced to below 200 meters, the NMC said.

Drivers have been asked to maintain safe speeds. Airports, freeways and ports have been instructed to take appropriate safety measures.

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