Health officials in the south east Asian state said the man undergoing tests had been at Singapore General Hospital but was now in isolation at the island's Tan Tock Seng Hospital.
Although it is believed to be an isolated case, the country has ordered 25 people, who may have had contact with the man, into quarantine.
But WHO spokeswoman Fadela Chaib said the medical student from Singapore had suffered a high fever but did not match the full case definition of the pneumonia-like disease.
"X-rays did not show up atypical pneumonia but other tests showed it could be SARS. These are conflicting results," Chaib told journalists.
It's SARS, says Singapore
Singapore health officials conducted two rounds of Polymerase Chain Reaction (PCR) tests. These amplify genetic material and are key diagnostic tools in testing for Severe Acute Respiratory Syndrome, or SARS. But the tests are not always reliable.
"None of us can predict what will happen later this year. Will SARS come back or not?"
"We have a patient who has fever, and we've done serology and PCR tests that were all positive," said Khaw Boon Wan, acting minister for health
"Regardless of what the WHO's definition is, I can't ignore the fact that this is a SARS case."
"I'm anticipating that the tests from the US will be positive because both our tests have been positive. I'll bet on CDC results coming back positive," the minister added.
Singapore's announcement of another outbreak of Severe Acute Respiratory Syndrome sent Asian markets tumbling as airline stocks nosedived on the news.
During Singapore's last outbreak, about 8000 people were quarantined. The island took aggressive measures to contain the disease, carrying out temperature checks at border points, hospitals and hotels, and installing closed-circuit TV cameras in homes to enforce the quarantine.
SARS originated in southern China and was spread early this year to 30 countries by travellers. It infected nearly 8500 people globally, and more than 800 died including 33 in Singapore.
It had a devastating impact on Asia's economies, costing billions of dollars in lost business and reduced growth. Travellers cancelled flights, consumers stayed at home and hotel rooms emptied.
Asian stocks crashed on news of
a possible new SARS case
In Singapore, SARS triggered the nation's biggest-ever economic contraction in the April to June quarter.
The WHO had declared the global outbreak contained on 5 July. It took Singapore off a list of SARS-affected regions on 31 May, about three weeks after its last patient was isolated.
On Monday, WHO officials warned health specialists in Manila of a possible resurgence of the disease and urged nations to boost surveillance.
"None of us can predict what will happen later this year. Will SARS come back or not?" Director-General Lee Jong-wook told a five-day regional WHO meeting.
"We have to prepare on the assumption that this will come back. Our challenge now is to enhance surveillance networks that will detect and deal with SARS if it does come back."
WHO officials have said they are not sure if SARS, which has no known cure, is a disease confined to winter months.