Thick fog disrupted traffic in parts of north China Monday, provincial and municipal authorities said.

The affected areas are Shandong and Liaoning provinces, Tianjin Municipality and Ningxia Hui Autonomous Region.

The meteorological station of Ningxia said Monday nine flights were delayed, stranding 600 passengers in an airport in Yinchuan City, capital of northwestern Ningxia Hui Autonomous Region.

The station also issued an alert for thick fog with a visibility of less than 50 meters in Yinchuan and Shizuishan cities.

Visibility in most parts of Tianjin fell to between 100 to 200 meters Monday, said Zhao Gang, chief forecaster of Tianjin's meteorological station.

Nine of the 12 expressways into Tianjin were closed, said a spokesman with the city's traffic police.

Tianjin's traffic authorities were well-prepared for the situation and no serious congestion was reported, the spokesman added.

Also closed were 10 expressways in northeastern Liaoning Province, said the provincial expressway management authorities.

Fog was shrouding central and west Liaoning where the visibility was as low as 50 meters, said a spokesman with the meteorological station of Shenyang, capital of Liaoning Province.

Shandong Hi-Speed Group in eastern Shandong Province said in a statement that 30 toll stations on seven highways were closed Monday due to the fog.

The visibility dropped to less than 200 meters in the heaviest-shrouded areas in Shandong, said the provincial meteorological station.

The traffic authorities urged motorists to drive slowly or stop at service centers.

Meteorological authorities suggested patients with respiratory, heart or lung diseases stay indoors.

The visibility was from 50 to 200 meters in Changchun, capital of northeast China's Jilin Province, which had not affected the traffic as of Monday night, local authorities said.

The visibility is predicted to be less than 1,000 meters in most parts of north China's Hebei Province, and it will be 200 meters in some parts, from late Monday to early Tuesday, according to the provincial meteorological bureau.

The fog was formed as the snow had been melting, increasing vapor in the air, said the bureau.