Frequent spells of smog that have hit Beijing four times since November, may make it difficult for China's capital to achieve its annual air quality improvement goal, the media reported on Monday.

Beijing has set a target of cutting the concentration of PM2.5-particulate matter with a diameter of less than 2.5 microns that can penetrate the lungs and harm health-by 5 percent year-on-year, China Daily reported.

Readings of the pollutants averaged 86 last year, and the figure was 74 for the first 11 months of this year.

But in recent days, when the city has twice been forced to issue the highest-level red alert warning for smog, the average reading was 241, more than three times the national safety level.

This will definitely make the annual average reading for PM2.5 higher and the pollutant-reduction target harder to achieve, said Zhang Dawei, head of the Beijing Municipal Environment Monitoring Centre.

On Friday, Beijing issued its second red alert, which helped to reduce hazardous pollutants by up to 30 percent, the municipal environmental watchdog said.

The red alert took effect at 7.00 a.m. on Saturday and will remain in force until Tuesday. Under the emergency measures, industrial operations have been suspended and the number of vehicles on the roads reduced by half based on odd and even license plate numbers.

The smog is forecast to last until Wednesday when a strong cold front is due to sweep across northern China.