Thailand ruled out on Wednesday Zika as the cause of two cases of microcephaly but is investigating two other cases that could be the first linked to Zika in Southeast Asia of the condition that results in babies being born with small heads.

Microcephaly is a rare birth defect that has been linked, in Brazil in particular, to the mosquito-borne Zika virus, which is spreading in Southeast Asia.

Thai health authorities said on Tuesday they were investigating four suspected cases of Zika-related microcephaly in three babies and a 36-week old unborn baby.

But on Wednesday, Apichai Mongkol, director-general of the ministry's Department of Medical Sciences, said Zika had been ruled out as the cause in two of the cases.

"The test results showed that two did not have the Zika virus but it is still unclear with the other two because we found three flavivirus which means it could be dengue, Zika or meningitis," Apichai told Reuters.

"To be sure which of the three they contracted we need to investigate further."

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There are 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 World Health Organization (WHO).

It was not clear which of the four cases had been ruled out as having a link to Zika.

Zika has been linked to more than 1,800 confirmed cases of microcephaly in Brazil but there have been no confirmed cases of Zika causing the condition in Southeast Asia, according to the WHO.

Microcephaly has several potential causes including various infections, exposure to toxins and malnutrition, the WHO says.

Health authorities in Southeast Asia have reported a rising number of Zika cases with Thailand confirming 349 since January, including 33 pregnant women. Singapore has recorded 393, including 16 pregnant women.

The Philippines, Malaysia and Vietnam have also reported Zika cases.

Thailand's public health ministry said earlier the four suspected microcephaly cases were not in Bangkok but it declined to say where they were found.

The Bangkok Metropolitan Authority said on Wednesday officials had confirmed 26 Zika cases in the city this month.

The director-general of the Disease Control Department, Amnuay Gajeena, said out of 33 cases of pregnant women with Zika, eight had given birth without complications.

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.

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.