(Correct paragraph 3 number of cities to 10, not 8; corrects paragraph 4 alert status to red, not orange)
BEIJING, Dec 16 (Reuters) - Steelmakers in China's top province for producing the construction material are being forced to crimp output after the local government said they had to reduce emissions in the wake of a series of pollution alerts.
The world's No.1 steelmaking nation has been battling environmental degradation left by decades of breakneck economic growth, adopting a colour-graded warning system in a crackdown on smog.
Ten cities in the province of Hebei, which surrounds Beijing, have been issued with 'red alerts' for smog, the highest possible air pollution warning.
The city of Tangshan, home to a quarter of Hebei's steel factories, has also been given a first-level 'red alert'.
Factories in cities with red alerts must reduce emissions by 40 percent, while those under orange alerts have to curb them by 30 percent, the provincial environmental bureau said in a statement on Thursday. It was unclear exactly how much steel output would be impacted.
Heavy smog is due to last until Wednesday, when the alerts are likely to be lifted along with the emissions curbs.
China have advised 23 northern cities to issue red alerts in the face of the "worst" smog the country has experienced since autumn, state media said.
Reporting by Muyu Xu and Josephine Mason; Editing by Joseph Radford