TIANJIN - Hundreds of flights have been canceled in North China's port city of Tianjin because of the heavy smog that limited visibility on Sunday.

The municipal government has issued an orange alert for smog and haze on Saturday night. An orange alert means heavy pollution -- PM2.5 higher than 150 -- for three consecutive days.

A total of 309 flights have been canceled, 15 forced to land in other airports and one returned as of 3 p.m., according to the airport. Emergency plans have been activated to take care of the stranded passengers.

All highways in the city have also been closed, the city's transport authorities said Sunday afternoon.

Local weather bureau forecast the ongoing smog will continue till Jan. 5 when a cold haze will disperse the pollution.

Tianjin is a frequent victim of smog that is common in the winter in north China, where cold weather and burning of the dirty coal for heating combine to exacerbate the situation.