Severe smog that has shrouded the northern Chinese city of Tianjin has grounded flights and closed highways, local transportation authorities said.

Tianjin's weather bureau issued an orange alert -- the second highest in a four-tier system -- for air pollution on Sunday morning, warning that pollutant density would hit a peak through Monday. At 8:30 p.m. Sunday, Tianjin Binhai International Airport was closed for flights due to low visibility.

As of 11:30 a.m. Monday, 181 flights had been cancelled at Tianjin Binhai International Airport, with another 59 delayed. All highways in and out of the city, except one, were closed due to extremely low visibility.

Meanwhile, Dalian, another city in North China, issued an orange alert for massive delay on Monday morning due to heavy smog. As of 11:30 a.m. Monday, a total of 128 flights had been cancelled or delayed at Dalian Zhoushuizi International Airport.

More than 20 cities in northern China, including the capital Beijing, have issued smog alerts since Friday night.

Meteorological authorities said the smog would not disperse until Wednesday.