BEIJING, April 10 -- China's national observatory on Wednesday renewed a yellow alert for thick fog in some parts of the country.

On Wednesday, dense fog is expected to shroud parts of the Yellow Sea, the East China Sea and coastal waters off the Shandong Peninsula and Hangzhou Bay, as well as Jiangsu and Zhejiang provinces, according to the National Meteorological Center.

Visibility in some affected areas will be reduced to less than 1 kilometer, the center said.

Vessels engaged in sailing and other operations have been advised to strengthen their lookout, regulate speed and maintain a sensible distance to ensure safe navigation.

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