Singapore on Friday reported three more coronavirus cases that have not been linked to previous infections or travel to China, prompting it to raise its alert to orange, the same level it reached during the SARS outbreak in 2003.
Singapore now has 33 confirmed coronavirus cases.
The orange alert means the virus is severe and passes easily from person to person and was last raised during an outbreak of H1N1 influenza in 2009-2010.
Severe Acute Respiratory Syndrome (SARS) killed more than 30 people in Singapore and hundreds worldwide.
"As there are now a few local cases without any links to previous cases or travel history to China, we have stepped up our risk assessment," the health ministry said in a statement, adding that firms should be prepared for "widespread community transmission".
The highest alert is red, which indicates the virus is spreading widely.
Singapore advised businesses to cancel or defer non-essential large-scale events but said the Singapore Airshow will go ahead next week although organisers may limit public visitors.
Of the new cases reported on Friday, one was a teacher and authorities said all students and staff who had come into contact with her would have to spend two weeks at home.
A British man has also contracted the coronavirus after travelling to a business meeting in Singapore which has also been linked to cases in the city-state, Malaysia and South Korea, health ministry official Kenneth Mak said.
Mak said authorities had not yet identified the source of infection at the meeting of more than 100 employees from an as-yet-unnamed firm at the Grand Hyatt Hotel in mid-January.