BEIJING -- Heavy fog continued to shroud north and central China on Wednesday, leaving expressways closed down and flights delayed.

As of 8:30 am, traffic police in Beijing had closed several sections of expressways, including roads linking the capital with Harbin in the northeast, Shanghai in the east, and neighboring Tianjin Municipality.

In Central China's Henan province, the visibility at Zhengzhou Xinzheng International Airport was only 50 meters on Wednesday morning, delaying flights. Some 180 flights at the airport were canceled over the past two days.

Many regions in China have been under heavy smog since Friday, disrupting traffic. Beijing on Sunday extended its orange alert for heavy air pollution until midnight on Wednesday.

China's national observatory on Tuesday issued a red alert for fog and renewed an orange alert for smog in a number of northern, eastern and central regions.

China has a four-tier color-coded warning system for severe weather, with red being the most serious, followed by orange, yellow and blue.