BEIJING, April 27 -- China's national observatory on Saturday evening issued a yellow alert for heavy fog in some parts of the country.

From Saturday evening to Sunday morning, thick fog is expected to shroud parts of the Bohai Strait, the coastal waters of the Liaodong and Shandong peninsulas, the coastal waters of Jiangsu, and parts of the Yellow Sea.

Visibility in some affected areas will be reduced to below 1,000 meters, the center said.

It has advised motorists to control their speeds to ensure safety, and said that precautionary measures should be taken at airports, on expressways and at ferry terminals to ensure traffic safety.

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