The heavy pollution registered on Sunday in Beijing and other parts of northern China forced authorities to cancel or reroute dozens of flights to and from the capital as well as close dozens of roads packed with vacationers returning from their National Week holiday.

State-run CCTV reported that the Beijing airport cancelled 13 departing flights, rerouted 29 others to nearby cities and directed five more to return to their cities of origin.

Among the highways shut down by the government were main ones connecting Shanghai with the capital and with Harbin in northern China, a move that the official Xinhua news agency reported is making it difficult for thousands of Chinese to return to their homes after their week-long vacation.

The poor atmospheric conditions in many cities of northern China were said to be due to a "dense fog" but also to high pollution, which apparently was not mentioned publicly by Chinese authorities until after the U.S. Embassy did so.

The U.S. Embassy provides daily measurements of air quality in Beijing and said early on Sunday that the city was experiencing "dangerous" level of pollution.

The Beijing Meteorological Center then added its own warning to the U.S. alert, saying that the capital was experiencing the maximum pollution level on its six-point scale.

Visibility in the capital was reported to be less than 1,000 meters (3,280 feet), while at the airport it was further reduced to between 250 and 500 meters (820 and 1,620 feet). EFE