The national environmental authority has launched a new plan that targets Beijing, Tianjin and four neighboring cities with tough restrictions in an effort to curb severe smog in the northern region.

The six core cities will have a unified response against severe air pollution by March 15, the first step in coordinating regional efforts to determine alert levels and issue responses, said Chen Jining, minister of environmental protection, on Thursday.

In addition to the capital of Beijing and the northern major port hub of Tianjin, the cities are Baoding, Langfang, Tangshan and Cangzhou in Hebei province, all of which have frequently seen severe smog since 2013.