A cloud of pollution descended over Beijing at the weekend, shrouding the city and its famous cultural landmarks in a thick haze amid a US warning against physical activity outdoors.

The Beijing Municipal Environmental Monitoring Center said on its website Sunday that pollution levels in the city's six core districts was at 225-245.

According to a table carried in the state-run Beijing News daily, such a reading corresponds to Level 5 on the pollution scale. Anything above 300 is Level 6, China's highest.

Readings posted by the United States embassy, however, were much higher.

In an email message to American citizens Sunday morning, the embassy said that readings on its Air Quality Index (AQI) "have averaged over 300 in the 24-hour period beginning at 8:00 pm on October 4, and were over 400 overnight".

The embassy added that based on recommendations by the US Environmental Protection Agency (EPA), AQI levels surpassing 301 "are considered hazardous" with the EPA recommending that amid such readings "everyone should avoid all physical activities outdoors".

A photo on the front page of the Beijing News showed the city's Forbidden City -- once home to China's emperors and a major tourist site -- enveloped in thick haze on Saturday.

The pollution comes as China's annual Golden Week national holiday approaches its final day Monday and while Beijing and environs are hosting major international sporting events.

The final of the China Open tennis tournament was set for Sunday in Beijing, while the Reignwood LPGA Classic women's golf tournament was scheduled to enter its final round.

Separately, Sunday also marks the opening ceremony for the East Asian Games athletics meet in Tianjin, 135 kilometres (84 miles) southeast of the capital.

Cities across China have been hit by intense air pollution in recent years, much of it caused by emissions from coal-burning power stations, with levels of small particles known as PM2.5 reaching as high as 40 times World Health Organization limits this year.

China's pollution problems are blamed on rapid urbanisation, dramatic economic development and climatic factors. Pollution tends to worsen as winter approaches. Earlier this year, pollution levels soared with the US embassy's AQI soaring above 500.

Pollution has been linked to hundreds of thousands of premature deaths, and has tarnished the image of Chinese cities including Beijing, which saw an almost 15 percent drop in tourist visits during the first half of this year.

China vowed in September to reduce levels of atmospheric pollutants in Beijing and other major cities by as much as 25 percent to try to improve their dire air quality.

China's State Council, or cabinet, said at the time that "concentrations of fine particles" in the capital's air will fall by "approximately 25 percent" from 2012 levels by 2017.