Heavy smog and fog on Sunday made Beijing the most polluted of 47 cities monitored nationwide, according to the official environmental watchdog.

Beijing is shrouded in drizzle and heavy fog on Sunday afternoon, causing low visibility and slow traffic in downtown areas. [Zou Hong/China Daily]

By midday on Sunday, Beijing's air quality was rated as "poor" by the China National Environmental Monitoring Center - just one level away from "hazardous".

The capital was the only city listed in this category of the 47 covered by the center.

The center classifies air quality in China's urban areas from levels one to five: excellent, fairly good, slightly polluted, poor and hazardous.

According to the Beijing municipal environmental protection bureau's website, air quality in Huangcun in the capital's Daxing district and Liangxiang in Fangshan district had already been classified "hazardous" by midday on Sunday.

The US embassy in Beijing also qualified the air near its compound in Chaoyang district as "hazardous" in its own measurements made available on the popular micro-blogging site, Twitter.

"Overnight rain and wind will help carry away dust particles, which is the major pollutant," Du Shaozhong, deputy director of the bureau, told China Daily late on Sunday.

"We might have a better day tomorrow, but it's not for sure."

Wang Xiaoming, another senior bureau official, told China Radio International on Sunday that fog had started to accumulate in Beijing since Oct 7 and had stayed in the city due to calm weather.

"The air quality then started to drop sharply after pollutants filtered into the fog. The city's air quality has been below the national standard (fairly good) for four days since Oct 7," he said.

The China Meteorological Administration issued a nationwide heavy fog warning at 6:00 am on Sunday.

It said heavy fog will obviously affect air quality and advised residents to take appropriate measures to protect themselves from potential health hazards caused by pollution. It also warned drivers to reduce speed.

Eleven provinces and regions, including the capital city, were affected, it said, with visibility reduced to 100 meters in some areas causing road accidents.

The heavy fog warning was withdrawn 12 hours later "as cold fronts arrived, but moderate fog will last until Monday morning in some areas", the administration said.

At least 32 people have been killed in traffic accidents over the past few days as heavy fog shrouded huge swathes of the country.

At least 17 people were killed and six were seriously injured after a passenger coach collided with a cement truck in Jiangsu province, local authorities said on Sunday.

The accident, which occurred on Saturday morning, was on the highway between Hefei, capital of Anhui province, and Nanjing, capital of Jiangsu, in thick fog. The truck caught fire before crashing into the coach.

The passenger coach was carrying 53 people, exceeding its capacity of 47, officials said.

Rescue services took 23 injured passengers to hospital, including six who were in a serious condition.

The accident followed a multi-vehicle crash in Henan province on Friday morning, in which eight people were killed and dozens injured when up to 30 vehicles were involved in a pile-up in thick fog.

On the same day, seven people died in six accidents in Anhui province, also on fog-shrouded highways.