BEIJING -- Beijing, Tianjin and eight cities in Hebei province issued a yellow smog alert on Saturday.

Smog is forecast for central and southern parts of the Beijing-Tianjin-Hebei area over the weekend, clearing up from Monday. During the period, the air quality index could exceed 300.

Factories will take emergency measures to reduce emissions while construction sites must control dust.

Beijing failed to meet a key pollution reduction target last year with annual average density of PM2.5 down only four percent, less than the five percent target, the city's mayor said on Friday. Hebei and Tianjin did a better job.

The average density of PM2.5 last year fell by 12 percent in 11 cities in Hebei and the average reading dropped by 13.5 percent in Tianjin.