SHENYANG, Nov. 21 -- A thick smog shrouded northeast China's Liaoning Province, disrupting highway traffic and flights on Friday morning.

Visibility in some areas in Liaoning were less than 20 meters as the provincial meteorological authorities issued a red alert, the highest, four times for the weather.

Eleven highways in Liaoning were shut down completely and ten others were closed partially.

The smog also forced the Shenyang Taoxian International Airport to be closed at 7 a.m. Friday, delaying 18 flights. It reopened three hours later.

The National Meteorological Center maintained its "yellow" warning, the second-lowest alert on the four-level scale, as the smog continues to shroud Beijing and Tianjin municipalities as well as Hebei, Henan and Shandong provinces on Friday.

A cold front, which will be in force later on Friday, will help disperse air pollution and improve visibility in these regions, the observatory said.