The number of confirmed Zika cases in Singapore surged past 100 late on Wednesday, including the first pregnant woman to be infected by the disease which can cause deformities in babies.
The United States and Britain joined Australia and Taiwan in advising pregnant women to avoid non-essential travel to the city-state, while a local health expert warned the infection rate would rise.
The Ministry of Health and the National Environment Agency said in a joint statement late Wednesday they had identified 24 new infections plus nine more detected as a result of testing previous cases, bringing the total to 115.
A pregnant woman was among those who tested positive for the virus.
"Her doctor is following up closely with her to monitor her health and the development of her baby," the statement said.
"She will be referred to a maternal-foetal medicine specialist for counselling and advice."
The Aedes mosquito-borne Zika, which has been detected in 67 countries and territories including hardest-hit Brazil, causes only mild symptoms for most people, such as fever and a rash.
But in pregnant women it can cause microcephaly, a deformation in which babies are born with abnormally small brains and heads.
Brazil, where the disease has become an epidemic, has reported 1,835 cases of microcephaly, the World Health Organization says.
Singapore on Wednesday urged pregnant women showing symptoms of fever or rashes to get tested.
Mosquito breeding
Expectant mothers with male partners who have tested positive were also told to visit their doctors.
The environment agency also said it would expand its operations to eradicate mosquitoes and their breeding places at a potential new cluster in the suburb of Bedok North and urged residents to cooperate, warning it will enter "inaccessible premises" by force if necessary.
"Over time, we expect Zika cases to to emerge from more areas," Health Minister Gan Kim Yong said in the statement.
"We must work out a plan on the basis that there is Zika transmission in other parts of Singapore."
Despite the rise in Zika cases, a spokesman for the Singapore Grand Prix told AFP Wednesday the Formula One race will go on as scheduled from September 16-18.
"We will continue to work with all relevant government agencies and implement any recommended precautionary measures as directed or deemed necessary," he said.
Infections set to rise 
Since Singapore reported its first locally transmitted Zika infection on Saturday, confirmed cases have soared as authorities ramped up testing.
Leong Hoe Nam, an infectious diseases specialist at Singapore's Mount Elizabeth Novena Hospital, said the number is likely to climb.
"There are very few mosquitos carrying the Zika virus but you must remember (that) for every one Zika case found, four more are asymptomatic," he told AFP.
But he said Singaporean authorities -- who spend some Sg$1.0mn ($733,000) a day on exterminating mosquitoes -- were in a good position to deal with the outbreak.
"We have a very determined government with the funds... and a rather obedient population," he said. "When told to stay home, people do stay home and follow instructions."
Despite being one of Asia's cleanest cities with high health care standards, Singapore is a densely populated tropical island with heavy rainfall. It has a chronic problem with dengue fever, spread by the same Aedes mosquito that carries Zika.
Singapore's first Zika case came in May from a man who had visited Sao Paulo in Brazil.