Singapore health authorities have warned the Zika virus, which has been linked to a rare birth defect, is spreading in the affluent city-state.
The Ministry of Health on Sunday confirmed 41 cases of locally-transmitted Zika, including 36 foreign construction workers.
Authorities said they had identified "other areas of concern" outside the Aljunied Crescent and Simms Drive areas on the island's south east where the 41 cases were confirmed.
On Saturday, authorities confirmed the first local transmission of the virus to a 47 year-old Malaysian woman, also from the Aljunied Crescent area.
The Ministry of Health warned in a statement that it cannot rule out "further community transmission in Singapore since some of those tested positive also live or work in other parts of Singapore."
"We expect to identity more positive cases," it said.
None of those confirmed to have the virus have travelled to Zika-affected areas recently.
"This confirms that local transmission of Zika virus infection has taken place," the ministry said.
Those affected include a 65-year-old Singaporean man and his 21-year-old son, a 30-year-old Singaporean man who works at a construction site and an unemployed 44-year-old Singaporean man.
Zika was detected in Brazil last year and has since spread across the Americas, including parts of the US.
The virus has also been detected in other Asian nations, including Bangladesh, Thailand, Malaysia, Vietnam, Laos, Malaysia, Maldives and the Philippines, according to the World Health Organisation.
But the Singapore outbreak appears to be the largest 'cluster' discovered in Asia in the current epidemic.
The virus has mild affects on most people but can be fatal for unborn children.
About four in five people affected by Zika show no symptoms. For those who do, symptoms include viral fever, skin rashes, body aches and headache.
Pregnant women who get infected may give birth to babies with small heads, a condition called microcephaly, and other brain defects.
Zika is spread by the Aedes mosquito, which also spreads dengue.
Singapore's National Environment Agency has deployed more than 200 workers to the affected areas, where they are spraying insecticide and cleaning drains.
The agency has the authority to enter buildings by force if necessary to destroy habitats where mosquitoes can breed.
Local residents welcomed the clean-up.
"I'm very scared of mosquitoes because they seems to bite me, they never bite my husband," Janice, 31, who only gave her first name, told Reuters.
"This concerns me because maybe in a couple of years I want to have another (child)."
Malaysia has stepped up surveillance at main transit points with Singapore, where Zika prevention leaflets are being handed out.
Malaysia said earlier this month it had screened more than two million visitors at air, sea and land entrances but found no Zika cases.