BEIJING, Dec. 12 -- Several Chinese cities closed expressways and canceled flights as heavy fog and smog continue to grip a large part of the country but showing signs of easing on Sunday.

Almost all expressways in the eastern province of Jiangsu were closed on Sunday morning. Traffic resumed on certain roads in the afternoon but remained slow, local traffic authority said.

In Nanjing, capital of Jiangsu, visibility dropped to below 50 meters in some areas, and the airport reported delays or cancellation of more than 100 flights.

Traffic was also temporarily suspended on several expressways in Beijing, which became smoggy on Saturday. The bans had been lifted as of 2:40 p.m. as winds picked up.

The coastal city of Qingdao in Shandong Province reported dense fog on Sunday, with closures of several expressways and delays of over 50 flights.

The city also closed its port, and more than 70 ships have been stranded, according to Qingdao's frontier inspection station.

Large parts of China have seen foggy or smoggy weather since the start of December. National Meteorological Center on Sunday upgraded its smog alert to orange, the second highest in its four-tier warning system.

The smog has led to high pollution readings in many cities including Shanghai, where authorities banned outdoor school activities and limited construction work.

Even cities with usually good air quality records, including Fuzhou in southeast China's Fujian Province, reported medium to heavy pollution on Saturday and Sunday, suggesting how far reaching the smog has been.

The Ministry of Environmental Protection earlier this week blamed unfavorable weather patterns, vehicle exhaust and coal burning for winter heating for the problems.

According to the National Meteorological Center, the current wave of smog will persist in east China until Monday, though it will disperse in north China on Sunday.