Initial investigations by mainland officials have revealed the patients were farm workers who handled sick or dead pigs and sheep. Only one of those who contracted the illness has fully recovered.

Patients suffered fever, nausea, vomiting and haemorrhaging. Those who died went into shock before falling into a coma.

Hong Kong's Centre for Health Protection (CHP) issued a warning yesterday - a day after it was notified by the Ministry of Health of the outbreak, which is thought to involve up to 20 patients, aged 30 to 70, who lived in towns and villages in Jianyang city and Ziyang city's Yanjiang district.

The CHP informed the Hospital Authority, which alerted all the city's 14 accident and emergency departments 'to be vigilantly on the lookout for patients with similar symptoms'.

It is the first time an alert has been issued to public hospitals to be on the lookout for a mystery illness from across the border since the outbreaks of Sars and bird flu. The worry is that people travelling from the mainland could bring the disease to the city, as happened with Sars.

The spokesman said the CHP was maintaining close communication with the Ministry of Health, adding it was ready to offer expertise support.

A spokesman for the Food and Environmental Hygiene Department said Hong Kong did not import live pigs from Sichuan. The department believes that existing inspections and health certification for imported frozen meat are sufficient to prevent diseased meat entering the food chain in Hong Kong.

The Ministry of Health and Ministry of Agriculture have sent a joint expert team to Sichuan to assist in the investigation, treatment and control of the outbreak. The WHO has asked Beijing for more information about it.

Hitoshi Oshitani, who is responsible for communicable disease surveillance and response in the WHO's western Pacific region, said: 'It is not rare for unexplained outbreaks of diseases like these to appear, especially in less developed rural regions.'

Dr Oshitani said it was unlikely that the outbreak was of bird flu. He said: 'It would be foolish for us to speculate on the disease or diseases active in this situation.'

An infectious disease specialist, Lo Wing-lok, said the most dangerous possible cause of the Sichuan outbreak was anthrax. He named the pig streptococcus bacteria and Nipah virus as other possible causes.

'They should investigate the pigs, people, trace contacts and nail down the infection as soon as possible, otherwise more deaths might occur,' he said.

The CHP advises people to:

Protect themselves against insect bites;

Avoid exposing open wounds to any contaminated object;

Avoid contact with sick or dead animals and their excreta or bodily fluids;

Avoid contact with unknown chemicals; and

Observe good hygiene practices.

Returning travellers should consult doctors and report their travel history if they feel unwell, the centre said.