BEIJING, April 12 -- China's national observatory on Friday renewed a yellow alert for thick fog in some parts of the country.

On Friday, dense fog is expected to shroud parts of the Yellow Sea and coastal waters off the Shandong Peninsula, as well as parts of provincial-level regions including Henan, Hubei, Hunan and Chongqing, according to the National Meteorological Center.

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

Vessels engaged in sailing and other operations have been advised to strengthen their lookout, regulate their 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.