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

The city's weather bureau issued an orange alert - the second highest - for air pollution Sunday morning as a new bout of smog began. The bureau said the density of pollutants will reach its peak on Sunday and Monday.

Most air quality monitoring stations in the city warned about severe air pollution Sunday morning. Air quality index (AQI) readings at some points even exceeded 400.

As of 10 am Sunday, 35 flights had been delayed or canceled at Tianjin Binhai International Airport. All highways in and out of the city, except one, have been closed due to extremely low visibility.

The city's environmental protection department has strengthened inspection to control sources of pollution.

Over 20 cities, including Beijing, in north China have activated alerts and corresponding measures to address the smog that has hit the region since Friday night. Meteorological authorities said the smog will not disperse until December 21.