BEIJING - Beijing air control authorities has issued an alert for hazardous air and urged residents to stay indoors as the city choked by smog on Wednesday.

The city's office for severe air pollution emergency response issued a blue alert, the lowest of a four-level warning system, at 3 pm, for Wednesday and Thursday, blaming regional pollution and unfavorable weather conditions for the build-up of pollutants.

Beijing's air quality index hit 348 at 4 pm with the index of PM2.5, the smaller but more harmful particles, standing at 348, according to data on the website of the Beijing Environmental Protection Monitoring Center.

The office has ordered local pollution emitters to take further measures to cut emissions and advised residents to stay indoors and to wear masks when venturing out. Children, the elderly and those who suffer respiratory and cardio-cerebrovascular diseases are urged to reduce outdoor activity.

The air is forecast to gradually clear up on Thursday night when a cold front arrives.

Many cities in the neighboring regions, including the provinces of Hebei, Shanxi, Shandong and Liaoning, are also under heavy smog.

Prior to the smog, Beijing experienced an unusual two weeks of blue skies, with the help of cold fronts and a spate of measures, including traffic restrictions and factory closures in Beijing and surrounding regions, aimed at ensuring clean air for the APEC meetings.