Hazardous smog blanketing China's north-east has sparked more red alerts, with authorities advising residents in 10 cities to stay indoors, according to BBC News.

The announcement follows last week's warning that a vast area of China would be badly hit by pollution. Beijing saw its second red alert over the weekend. The latest wave of alerts includes the industrial port Tianjin.

Red alerts trigger advisories for people to stay inside, schools to stop classes, and restrict vehicle use.

An environmental ministry statement issued on Wednesday, December 23 night said the 10 cities with red alerts include Tianjin as well as smaller surrounding cities Puyang, Xinxiang, Dezhou, Handan, Xintai, Langfang, Hengshui, Xinji and Anyang.

They are among 30 cities including Beijing seeing "severe pollution". Another 20 cities have "heavy pollution".

On Wednesday, images of a purple-tinged sunset in Nanjing spread across social media with many users attributing it to the pollution in the city.