Statement by WHO comes amid “increasing evidence” virus to blame for surge in birth defects and neurological problems.
Authorities in Thailand have confirmed that two cases of babies with microcephaly, or abnormally small heads, were caused by the Zika virus, the first such cases found in Southeast Asia.
The health ministry said on Friday that tests remained inconclusive as to whether the third infant’s condition was connected to Zika.
“Two of the three infants [tested] had microcephaly due to the Zika virus,” Wicharn Pawan, a disease control official, told AFP news agency.
The World Health Organization this week said any confirmed link in Thailand would be the first identified in Southeast Asia.
Zika causes only mild symptoms in most, including fever, sore eyes and a rash.
But pregnant women with the mosquito-borne virus risk giving birth to babies with microcephaly – a deformation that leads to abnormally small brains and heads.
There is no cure or vaccine for the virus, which has infected more than 1.5 million people in nearly 70 countries since last year, according to WHO, with Brazil the hardest hit.
While Zika has been present in Southeast Asia for years, there has been an increase in the number of recorded cases in the region in recent months.
On Friday, WHO praised the reaction in Thailand where “authorities have been active in detecting and responding” to Zika, according to Dr Poonam Khetrapal Singh, WHO’s regional director for Southeast Asia.
This week Thailand’s health ministry said it was monitoring 36 pregnant women infected with Zika, three of whom recently gave birth to babies with microcephaly.
Prasert Thongcharoen, a Thai virologist, said Friday that “4.3 infants per 100,000”, are born with microcephaly in Thailand, twice the global average.
The condition can also be caused by Down’s syndrome and other infections during pregnancy such as German measles and chickenpox.
The announcement could affect Thailand’s key tourism industry, one of the kingdom’s few economic bright spots under junta rule, with many western governments warning pregnant women against non-essential travel to Zika-affected areas.
The tourism industry, which is expected to account for some 17 percent of Thailand’s GDP this year, has swiftly bounced back from coups, violent street protests and bomb attacks in the past.
Before the Thai test results were announced, the US Centers for Disease Control and Prevention (CDC) issued a travel advisory urging pregnant women to “consider postponing nonessential travel to Southeast Asia countries with reports of Zika virus”.
The news comes just one day before the start of Golden Week, an annual Chinese holiday that sees a flood of tourists descend on neighbouring Thailand.
The kingdom is hoping to receive 33 million visitors this year, boosted by surging numbers of visitors from China.