Bangkok (AFP) - Thai doctors are running tests to see whether Zika was the cause of microcephaly in two babies whose mothers are infected with the mosquito-borne virus, a health official said Wednesday.
Two other infants with the condition were cleared of a link to the virus.
If the remaining pair are found to be linked, they could be the first case of Zika-related microcephaly in Southeast Asia, according to the World Health Organisation.
"It is too early to conclude if (the other two) had Zika, we need more information," said Apichai Mongkol, director general of the Department of Medical Sciences.
Zika causes only mild symptoms in most, including fever, sore eyes and a rash. But pregnant women with the virus risk giving birth to babies with microcephaly -- a deformation that leads to abnormally small brains and heads.
Thai authorities have been monitoring 36 pregnant women infected with the virus.
Eight have now given birth and three of the babies have been born with microcephaly, while an ultrasound scan on another mother showed her child also has the condition, according to a ministry statement on Tuesday.
Between 200 to 300 Thai children are born with the condition each year, which can also be caused by Down syndrome and other infections during pregnancy such as German measles and chickenpox, the statement said.
"If a case of Zika-related microcephaly is confirmed then it would be the first confirmed case in the WHO Southeast Asia region," said Michael Vurens van Es, World Health Organization regional spokesman.
He said evidence suggested that Zika has been present in Southeast Asia for several years, adding that the number of confirmed cases has risen in conjunction with heightened surveillance.
Scientists this month warned that the world should prepare for a "global epidemic" of microcephaly as the Zika virus takes root in new countries.
Thai authorities have been at pains to reassure locals and the millions of foreign tourists who visit each year that they are fighting the spread of the mosquitos which carry the virus.
Experts say it will be years before a vaccine is developed to prevent infection from Zika.