Thirty-four patients have fully recovered from Zika virus, according to Singaporean authorities, while the health ministry has said it "cannot rule out" further cases.
Singapore confirmed 41 locally transmitted cases of the Zika virus on Sunday, although more are likely. A day after announcing its first locally transmitted case, that of a 47-year-old female Malaysian resident, the government reported 40 more.
The majority of victims of the virus were foreign workers based at a construction site.
All 41 are residents or workers in the Aljunied Crescent and Sims Drive area, the Ministry of Health and the National Environment Agency said in a joint statement. The virus can cause microcephaly in unborn babies.
"They are not known to have travelled to Zika-affected areas recently, and are thus likely to have been infected in Singapore," the authorities said. "This confirms that local transmission of Zika virus infection has taken place."
However, since Zika is spread by the Aedes mosquito, which is suspected to be active beyond the Aljunied neighborhood, the health ministry "cannot rule out further community transmission... since some of those tested positive also live or work in other parts of Singapore", the ministry said.
An Aedes aegypti mosquito inside a test tube is studied as part of research on preventing the spread of the Zika virus and other mosquito-borne diseases.
Singaporean authorities said they tested 124 people, primarily foreign construction workers employed on a site in the same part of Singapore. That site has been ordered to halt work, and workers' dormitories are being inspected.
Seventy-eight people tested negative and five cases were pending. Thirty-four patients had fully recovered. Four Singaporean men had developed symptoms of the virus in the past week and were hospitalised on Saturday.
Singapore hosts a large contingent of workers from the Asian subcontinent.
Singapore's Health Minister Gan Kim Yong told local media more imported cases are likely because Singapore is an international travel hub.
And because many Zika carriers display only mild symptoms or no symptoms at all — meaning infected people may not seek treatment — local transmission of these imported cases "is also very high," he said.
Health officials said the current infections came to light after a local clinic reported an unusual rise in viral fever on August 22.
This prompted a check by health ministry experts, who told doctors to refer new cases to the government-run Communicable Diseases Centre.
The World Health Organization said in a statement on Sunday that it did not know "which lineage of Zika is circulating" or "what the level of population immunity is to this lineage of Zika in Asia."
"It is important for countries to remain vigilant through surveillance for cases, to continue vector control, to inform people about Zika and how they can protect themselves, and to have the health system ready to supply the services needed to prevent and manage Zika and its consequences," the group told Reuters.
The mosquito-borne Zika virus usually gives rise to mild symptoms in adults such as a low fever, headaches and joint pain. It is also connected to brain damage and physical development problems in newborn babies, this condition is known as microcephaly.
The name of the virus actually comes from Uganda's tropical Zika forest, where the virus was first discovered in 1947.
Singapore deployed about 200 NEA officers to clean drains and spray insecticide in the mainly residential area early on Sunday to counter mosquito breeding grounds, and volunteers and contractors handed out leaflets and insect repellent.
All medical institutions in Singapore had been asked "to be extra vigilant" and immediately report any Zika-associated symptoms to the health ministry.
The government has employed pest control teams to spray insecticide in the Aljunied neighborhood in Singapore in an effort to kill mosquitoes.
Singapore said there were "ongoing local transmission" cases in Indonesia, Thailand and Vietnam.
Other countries in the region that have detected the Zika virus since 2013 include Bangladesh, Cambodia, Laos, Malaysia, Maldives and the Philippines, according to the WHO.
Malaysia said on Sunday it stepped up surveillance at main transit points with Singapore — handing out leaflets on Zika prevention and having paramedics ready to handle visitors with potential symptoms of the virus.
In Thailand, where close to 100 cases of Zika have been recorded across 10 provinces this year, the Department of Disease Control was screening athletes returning from the Olympic Games in Brazil, but was not otherwise changing its prevention measures.
"Every country in this region has Zika transmission cases," said Prasert Thongcharoen, an adviser to the DDC. "Thailand has, however, managed to contain the problem through early detection."
A Foreign Ministry spokesman said Indonesia was "following developments". Oskar Pribadi, a Health Ministry official, said there had been no recent Zika cases in the country.
Vietnam has to date reported three cases of locally transmitted Zika infection.
The current strain of Zika sweeping through Latin America and the Caribbean originated in Asia, where people may have built up greater immunity.