The official China Daily on Monday said a 35-year-old man, isolated in hospital after showing symptoms of the respiratory disease, was under observation in the southern city of Guangzhou in Guangdong province.
Two other SARS cases, one of them confirmed, have been treated there.
The man, who has not been formally declared a suspected case, had apparently had no contact with SARS patients or with wild animals that might be carrying the potentially lethal virus.
But doctors in Guangdong province, bordering Hong Kong, said chances were high that he would be formally designated a suspected SARS case, the Xinhua news agency said.
He was moved to the Number Eight People's Hospital, one of three hospitals in Guangzhou designated for SARS treatment, from a hospital under the local railway authority, it said.
A health worker in Guangzhou
disinfects dogs as a precaution
It was not clear, however, if the patient worked there or had anything to do with the railway.
Medical experts distinguish between a person being observed for SARS symptoms and a suspected SARS case.
Designation as a suspected case depends on observation, a string of tests, which could take several days and consultation between local medical authorities and the Health Ministry, the World Health Organisation (WHO) said.
The WHO also said it was asking the Health Ministry and local authorities about rumours of a possible fourth SARS case under observation in the boomtown of Shenzhen, directly across the border from Hong Kong.
A spokeswoman for Shenzhen's health department said she had not heard of a possible fourth case. "We do not know of any SARS-related case in Shenzhen," she said.
Last Monday, China confirmed its first case of Severe Acute Respiratory Syndrome - a 32-year-old television producer named Luo - since a world outbreak was declared over in July.
Luo has since recovered and left hospital last week, but health officials said a gene sample from him matched that of a coronavirus found in civet cats, a weasel-like animal eaten as a delicacy in southern China and sold in crowded markets.
Luo insisted he had had no contact with civets, but authorities ordered a cull of the animals in hope of averting an outbreak. A waitress from a restaurant that served civets is in hospital suspected of having the flu-like disease.
The virus first emerged in Guangdong, of which Guangzhou is capital, in late 2002 and went on to infect more than 8000 people in nearly 30 countries, killing about 800.