Experts say the dust picks up heavy metals and carcinogens as it passes over Chinese industrial regions, before hitting North and South Korea and Japan.

In Japan, the government urged drivers and train operators to be on alert because the sandstorms had reduced visibility.

The China Meteorological Administration warned of more sandstorms and dust for northern China, including Inner Mongolia and Hebei, from tomorrow until next week, Xinhua said.

At the weekend, the first sandstorms of the year affected Shanxi, Beijing and Hebei, which surrounds the capital, as well as Shandong province, state media said. Many have raised concerns about how Beijing's air quality will affect athletes at the Olympics later this year.

'We had to fight the yellow sand and strong wind with every step. We wore scarves and gauze masks to avoid being suffocated by small particles of sand and blowing grit,' Zhao Xin, a student at Taiyuan University of Technology in Shanxi, said.

'You could hardly open your eyes and the visibility was less than 10 metres. I prefer to stay in my dormitory all day when I don't need to go to class.'

Japan's Environment Minister Ichiro Kamoshita recently called on the mainland to disclose data on the yellow dust. According to Japan, Beijing has refused to release its own data and has insisted that any joint findings be kept from the public.

'I am not quite sure how and why it can be regarded as a national secret,' Mr Kamoshita said last month. 'Air is connected beyond national borders, and yellow sand travels beyond borders. I think it is important we share information.'

Japan, China and South Korea have begun joint research on the phenomenon. Japan's environment ministry recently began posting observation data and forecasts for dust waves on the internet.

South Korea issued a yellow dust warning at the weekend and major schools districts in southeastern regions urged parents to keep kindergarten and primary school children at home yesterday. 'We advised the closure because kindergarten, primary school students have weaker immune systems,' said Min Eyu-gi, an education official in Pusan.

South Korea blames the sandstorms, which originate in the Gobi Desert, for scores of deaths and billions of dollars in damage every year.

The state-sponsored Korea Environment Institute said the dust killed up to 165 South Koreans a year, mostly the elderly or those with respiratory ailments, and made as many as 1.8 million ill. Annual economic damage to South Korea from the storms was estimated at up to US$5.8 billion.

The mainland's meteorological administration said winds from the north would affect eastern China and parts of the south over the next three days, which could potentially carry dust to other parts of the country.

Air quality in Shanghai worsened on Sunday, prompting local media to warn that people suffering from heart or respiratory problems should remain indoors to avoid exposure to particulates.

However, a Shanghai weather official denied the sandstorms had hit the city, saying local conditions were partly to blame for the worsening air pollution.

'We can't say Shanghai has been directly affected by northern sandstorms,' Man Liping said.

Additional reporting by Fiona Tam in Shenzhen