Thailand confirmed on Friday that the Zika virus had caused two cases of microcephaly, a condition that results in babies being born with small heads, the first time it has been linked to Zika in Southeast Asia.
The confirmation came a day after U.S. health officials recommended that pregnant women postpone nonessential travel to 11 Southeast Asian countries, including Thailand, because of the risk of mosquito-borne Zika.
"We have found two cases of small heads linked to Zika, the first cases in Thailand," Prasert Thongcharoen, an adviser to the Department of Disease Control, told reporters in Bangkok.
He declined to say where in Thailand the cases were found.
The World Health Organisation (WHO) said in a statement the cases were the first of Zika-linked microcephaly in Southeast Asia.
U.S. health officials have concluded that Zika infections in pregnant women can cause microcephaly, a birth defect marked by small head size that can lead to severe developmental problems in babies.
The connection between Zika and microcephaly first came to light last year in Brazil, which has confirmed more than 1,800 cases of microcephaly that it considers to be related to Zika infections in the mothers.
Zika has spread extensively in Latin American and the Caribbean over the past year or so, and more recently it has been detected cropping up in Southeast Asia.
Thailand has confirmed 349 Zika cases since January, including 33 pregnant women, and Singapore has recorded 393 Zika cases, including 16 pregnant women.
Some health experts have accused Thai officials of playing down the risk of Zika to protect its thriving tourist industry but Prasert dismissed that.
"Thailand is not hiding anything and is ready to disclose everything," he said, adding that other countries in Southeast Asia might also have cases of Zika-linked microcephaly that they have not disclosed.
The U.S. Centers for Disease Control and Prevention (CDC) said on Thursday people should consider postponing travel to Brunei, Myanmar, Cambodia, Indonesia, Laos, Malaysia, Maldives, Philippines, Thailand, Timor-Leste (East Timor), and Vietnam.
The CDC has already issued a "travel notice" for Singapore, and said such a warning would be considered for the new countries if the number of cases rose to the level of an outbreak.
Thailand's confirmation of Zika-linked microcephaly comes ahead of China's week-long "Golden Week" holiday with Thailand expecting 220,000 Chinese visitors, up from 168,000 for the week in 2015, Tourism Authority of Thailand governor Yuthasak Supasorn told Reuters.
The Thai health ministry said on Tuesday it was investigating four suspected cases of Zika-linked microcephaly in three babies and an unborn baby.
The three babies were born with small heads but it was not clear from ultrasound results whether the 37-week unborn baby had a head size smaller than normal.
The ministry ruled out a link between Zika and microcephaly in two of the cases on Tuesday. But Prasert said tests had to be carried out again on one of those cases.
There is no vaccine or treatment for Zika. An estimated 80 percent of people infected have no symptoms, making it difficult for pregnant women to know whether they have been infected.
There are also no specific tests to determine if a baby will be born with microcephaly but ultrasound scans in the third trimester of pregnancy can identify the problem, according to the WHO.
Zika is commonly transmitted through mosquitoes but can also be transmitted sexually.
Another health ministry adviser urged everyone to work to stop the spread of mosquitoes but said people should not panic.
"Don't have sex with a Zika-infected person. If you don't know if they are infected, then use a condom," the adviser, Pornthep Siriwanarangsan, told reporters.
"We can't stop women from becoming pregnant ... but we mustn't panic."
Microcephaly in babies can lead to respiratory problems related to malformation of the brain, a very serious threat to the lives of babies in the first year of their lives.
Children with microcephaly face lifelong difficulties, including intellectual impairment.
Zika was first identified in Uganda in 1947 and was first isolated in Asia in the 1960s. It was unknown in the Americas until 2014.