Thirteen northern Chinese cities surrounding the capital Beijing have issued pollution alerts over the last few days, raising concerns that an industrial recovery in the region is increasing smog levels.

All 13 cities, including Tianjin and Tangshan, China's biggest steelmaking centre, had issued "orange" heavy pollution alerts by Sunday, the second-highest alert, the National Joint Research Center for Tackling Key Problems in Air Pollution Control (NJRC) said.

Air quality in the traditionally smog-prone region of Beijing, Tianjin and Hebei has improved markedly in recent years as a result of a "war on pollution" since 2014, which involved closing and relocating industrial plants as well as raising emission standards.

NJRC said the recent spike had been driven by an increase in industrial activity, with steel and cement plants operating at higher levels, and diesel truck traffic also rising. It expected the smog to persist until March 10.

China has been trying to re-energise its economy since lifting strict COVID-19 curbs at the end of last year, raising fears that pollution could be allowed to rise.

An orange alert means the three-day average air quality index (AQI) has risen above 200, classified as "heavy pollution", and normally triggers industrial closures or output cuts under Chinese regulations.

Though Beijing, where parliament is holding its annual meeting, has not issued an alert, its AQI hit 230 on Sunday night and climbed above 200 again on Monday.

Tangshan said on Saturday that it was beginning a 'Level 2' emergency response, the second time in a fortnight that it has implemented such measures.

During the first period, several steel mills were due to reduce their sintering by between 30% and 50% to meet the government requirements, consultancy Mysteel said in a report. (Reporting by Albee Zhang and David Stanway)