Beijing, Oct 15 (IANS): China's national observatory on Saturday morning renewed its yellow alert, the third-highest level of warning, for smog here.

Moderate smog will fall over Beijing and northeast Hebei province from Saturday morning to Sunday morning, with some areas to see heavy smog, Xinhua news agency cited the National Meteorological Centre (NMC) as saying.

Residents are advised to take protective measures and those with respiratory diseases should reduce outdoor activities, NMC said.

Beijing's Air Quality Index (AQI) hit 213 in Saturday morning, with hazardous fine particulate matter PM2.5 being the major pollutant, data from the municipal environmental monitoring centre showed.

The city has raised its alert for air pollution to yellow on Friday afternoon, with the AQI topping 300.

The weather is expected to improve starting Sunday evening with the arrival of a mild cold front, said the Beijing Environmental Protection Monitoring Centre.

China has a four-tier colour-coded system for severe weather, with red being the most serious, followed by orange, yellow and blue.

The meteorological authorities issue alerts when the AQI reaches 200 and severe air pollution is forecast to last at least one day.

The Ministry of Environmental Protection on Thursday said air quality in 338 Chinese cities showed improvement in the first three quarters of the year.

However, six out of the 10 cities with the worst air quality were still in Hebei province, an area adjoining Beijing, said the ministry.