Thailand is the worst-hit country in South-east Asia, with seven Zika cases uncovered between 2012 and 2014 in various provinces.
The Philippines, Cambodia, Indonesia and Malaysia each have had one Zika-related case since 2010.
But World Health Organisation (WHO) officials say the disease is likely to have spread far more widely in South-east Asia than the number of cases suggests.
This is partly because detection of the Zika virus is often delayed, its symptoms being similar to those of other mosquito-borne viruses such as dengue and chikungunya.
Thailand's most recent case was discovered earlier this month after a Thai man was stopped at a fever screening station at Taiwan's Taoyuan International Airport.
The Zika virus has "a widespread distribution" across Thailand, according to an article published in the American Journal of Tropical Medicine and Hygiene last year.
Authored by researchers from the Thai Health Ministry and the Armed Forces Research Institute of Medical Sciences, it detailed how a Canadian woman who travelled to Thailand in 2013 was diagnosed with Zika after returning home.
This prompted the health ministry to look deeper into blood test samples taken from patients with fever and/or rash.
It found seven Thai residents with the Zika virus from 2012 to 2014.
"These endemic cases, combined with previous infection reported in travellers, provide evidence that (Zika) is widespread throughout Thailand," said the report.
Zika is rarely fatal, with patients usually recovering on their own in about a week.
But there appears to be growing evidence in Brazil suggesting that pregnant women infected with Zika are at risk of developing babies with abnormally small heads, a condition known as microcephaly.
There has been no link so far between Zika and microcephaly in South-east Asia, said Dr Rana Bardan Jung from the WHO's Regional Office for South-east Asia.
WHO officials also say they have not come across Zika cases in China and India.
"But as this virus is spread by mosquitoes, I would say that, wherever there are mosquitoes, there's a possibility of this virus being found," said Dr Wichan Pawan, head of the Thai Health Ministry's risk communication bureau.