Medical teams descended on a Hong Kong apartment block on Tuesday to find out why a virus thought to cause atypical pneumonia has spread like wildfire, as fears grow it has mutated into an airborne infection.

Armed with sampling kits and wearing white surgical coats, caps, masks and gloves, investigators combed through Amoy Gardens apartments in crowded Kowloon, home to a third of all cases in Hong Kong, itself the city with the most cases outside mainland China.

But most of the residents had fled in fear, some violating a quarantine slapped on the area on Monday. Local police locked scores of remaining residents into their apartments as the Hong Kong government imposed drastic restrictions to combat the fast-spreading outbreak.

The desperate search came as U.S. researchers suggested the virus - thought to be a new species of coronavirus, a group that causes the common cold - may have mutated into a form that can be transmitted through the air, making it much more contagious.

"Under certain circumstances, at least, it may be airborne," Dr Julie Gerberding, director of the U.S. Centres for Disease Control - one of the world's leading infectious disease labs - told reporters in Atlanta.

"We are concerned about the possibility of airborne transmission across broader areas, and also the possibility that objects that become contaminated in the environment could serve as modes of spread," she said. "Coronaviruses can survive in the environment for up to two or three hours, so it's possible that a contaminated object could serve as a vehicle for transfer to someone else."

Scientists had previously believed that Severe Acute Respiratory Syndrome (SARS), as the pnemonia-type illness is called, was transmitted by coming into contact with droplets from a sneezing or coughing patient.

The ailment puts many of its victims into intensive care with very high fevers. Some of those who first contracted the disease have been cured with a cocktail of anti-viral drugs and steroids.

More than 1,700 people in 15 countries have been infected, and 62 have died - a death rate of around four percent so far. Canada has reported 31 new cases, but China - identified as the origin of the disease - has yet to report full figures as requested by the World Health Organisation, a United Nations body.

First case in Australia

Australia reported a first suspected case on Monday. The country's chief medical officer, Professor Richard Smallwood, told reporters in Canberra that a man who had returned from Singapore on February 12 had been admitted to hospital 11 days later with a fever, cough and difficulty breathing. The man, suspected of suffering SARS, has since recovered and did not infect others.

The urgency of the search at Amoy Gardens is to find whether the virus has mutated into an airborne threat. Doctors had thought the disease could be caught only through close contact with spray from patient's coughing, or bodily fluids. But over 110 people from only one tower block have caught SARS. More than 200 are believed to be infected in the whole housing estate.

"If it were related to water or sewage or air-duct systems, you can expect a continual appearance [of more victims]," said Professor Leung Ping-chung of the Prince of Wales Hospital, where the first Hong Kong patients appeared in March.

Coronavirus had been isolated in stool samples from the patients. Officials say one possible infection source may be a faulty waste pipe at Amoy Gardens, which had been spraying tiny clouds of waste matter in the direction of the worst infected block.

The Hong Kong government put residents of the block under quarantine in their own apartments on Monday, but hundreds had already fled, and some may be carrying the disease with them.

A government spokeswoman said officials were now hunting for those who had slipped the quarantine order. "We are trying to contact them on the phone. We have television advertisements asking them to contact us," she added.

Amoy Gardens is in a maze of crowded housing estates, markets and shops, and some residents from other blocks in the same estate had also fled overnight, fearing they would be next to be quarantined.

Officials in Canada, which has the most number of cases outside Asia, said the virus there had been restricted largely to medical staff and others who came into close contact with a handful of SARS victims who had flown to Toronto from Hong Kong.

"I can't say this often enough, the risk to the general public is extremely low," Dr Sheela Basrur, Toronto's chief medical officer, told a news conference.

Wave of panic setting in

The outbreak has sent a wave of panic across much of Asia, prompting people to cancel trips to the worst-hit countries and forcing a number of airlines to reduce flights. Some companies in Hong Kong are asking staff to work from home, or activating back-up centres in case their head offices are infected.

A number of foreign workers in Hong Kong, a former British colony which returned to Chinese rule in 1997, have departed quietly, taking their families with them on early Easter holidays. In Singapore, where some infections have also been found, nurses are attached to the airport to screen passengers arriving from areas hit by the virus.

The hope held out by doctors is that the virus's detailed makeup will be pinpointed shortly. Leung said some of the worst hit-patients in Hong Kong were successfully treated using antibodies in serum from recovered patients.

This suggests that those who have recovered may have developed some level of immunity. But researchers have not ruled out the possibility sufferers could be infected again and again.

Health staff in Hong Kong and other centres were particularly hard hit when the virus first appeared, but cases among them have fallen sharply in recent days, indicating that precautions in hospitals are working.

On Saturday, the World Health Organisation official who first identified the new disease, Dr Carlo Urbani of Italy, died from the infection. Urbani, 46, had been based in Hanoi, Vietnam, where he managed public health programs in the region. He first identified the disease in a American businessman who had been admitted to a hospital and later died.

Infections have now been reported in Germany, Italy, Ireland, Romania, Switzerland, the United States, Britain, France, Thailand, Vietnam and Taiwan, as well as Singapore, Canada, Hong Kong and mainland China, according to World Health Organisation.

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