Bangkok: Thailand has confirmed the first south-east Asia cases of microcephaly linked to the mosquito-borne Zika virus.

The confirmation of two cases of microcephaly, a birth defect marked by small head size, came a day after US health officials recommended that pregnant women postpone non-essential travel to 11 south-east Asian countries because of the risk of 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 but officials have said they were not in Bangkok.

The World Health Organisation (WHO) said the cases were the first of Zika-linked microcephaly in south-east Asia and the virus infection represented a serious threat to pregnant women and their unborn children.

"Countries across the region must continue to strengthen measures aimed at preventing, detecting and responding to Zika virus transmission," Poonam Khetrapal Singh, the WHO's regional director, said.

The connection between Zika and microcephaly first came to light last year in Brazil, which has confirmed more than 1800 cases considered to be related to Zika infections in the mothers.

Zika has spread extensively in Latin American and the Caribbean since then , and more recently it has been cropping up in south-east 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 this claim.

"Thailand is not hiding anything and is ready to disclose everything," he said, adding that other countries in south-east Asia might also have cases of Zika-linked microcephaly that they have not disclosed.

Thailand's confirmation of Zika-linked microcephaly comes ahead of China's week-long "Golden Week" holiday; Thailand is expecting 220,000 Chinese visitors, up from 168,000 for the week in 2015, Tourism Authority of Thailand governor Yuthasak Supasorn said. 

There is no vaccine or treatment for Zika. An estimated 80 per cent of people infected have no or few cold-like 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.

Microcephaly in babies can lead to respiratory problems related to malformation of the brain. Children with microcephaly face lifelong difficulties, including intellectual impairment.

Zika was first identified in Uganda in 1947 and first isolated in Asia in the 1960s.