Singapore has confirmed 41 cases of locally-transmitted Zika virus, mostly among foreign building workers, and says it expects more incidents to be identified.
All but seven of those infected had fully recovered, the country's health ministry and the National Environment Agency (NEA) said in a joint statement on Sunday.
The seven patients remained in hospital, the statement added.
On Saturday, authorities had confirmed a 47-year-old Malaysian woman living in southeastern Singapore as the city-state's first case of a local transmission of the virus - which has been linked to a rare birth defect.
The authorities said that they tested 124 people, primarily foreign workers employed on a construction site. Seventy-eight people tested negative and five cases were pending, while 34 patients had fully recovered.
Four Singaporean men had developed symptoms of the virus in the past week and were hospitalised on Saturday. It was not clear where the foreign workers were from or when their cases were detected. Singapore hosts a large contingent of workers from the Asian sub-continent.
None of those infected had travelled recently to Zika-affected areas. "This confirms that local transmission of Zika virus infection has taken place," the statement said.
The ministry "cannot rule out further community transmission since some of those tested positive also live or work in other parts of Singapore", the statement said.
"We expect to identify more positive cases."
Singapore, a major regional financial centre and busy transit hub, which maintains a constant vigil against the mosquito-borne dengue virus, reported its first case of the Zika virus in May, brought in by a middle-aged man who had been to Brazil.
All medical services in Singapore had been alerted "to be extra vigilant" and immediately report any Zika-associated symptoms to the health ministry.
Singapore deployed around 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.
Singapore said there were "ongoing local transmission" cases in Indonesia, Thailand and Vietnam. Other countries in the region to have detected the Zika virus since 2013 include Bangladesh, Cambodia, Laos, Malaysia, Maldives and the Philippines, according to the World Health Organisation.
Malaysian authorities said on Sunday that they had 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.
Vietnam has reported three cases of locally-transmitted Zika infection to date.
Zika, carried by some mosquitoes, was detected in Brazil last year and has since spread across the Americas. The virus poses a risk to pregnant women because it can cause severe birth defects. It has been linked in Brazil to more than 1,600 cases of microcephaly - where babies are born with small heads.
The WHO has said that there is strong scientific consensus that Zika can also cause Guillain-Barre, a rare neurological syndrome that causes temporary paralysis in adults.