Predominantly Buddhist Thailand will relax its strict rules against abortion to cover fetuses with proven birth defects linked to the Zika virus, health officials said on Thursday, doubling to 24 weeks a deadline for the procedure.

Thailand last week confirmed its first known cases of microcephaly linked to the mosquito-borne virus. The two cases of the birth defect marked by a small head were the first in Southeast Asia, following Zika outbreaks in the Americas.

Health experts who met this week to draft guidelines for expectant mothers with Zika concluded that abortions can be carried out at up to 24 weeks in case of serious birth defects.

"The difficulty with Zika is to determine microcephaly. It is usually found later in pregnancy," Pisek Lumpikanon, president of the Royal Thai College of Obstetricians and Gynaecologists, told Reuters.

"Legal medical abortions can be done up to 24 weeks," he added. "The reason is that at 24 weeks and after the baby already has a good chance of survival."

Abortion is illegal in Thailand, except in cases of rape or to save a woman's life or preserve her health, and if carried out in up to 12 weeks of pregnancy. Beyond that time, hospitals must decide on a case-by-case basis.

There are no specific tests to determine if a baby will be born with microcephaly but ultrasound scans can identify it in the third trimester of pregnancy, the World Health Organisation (WHO) says.

Thailand has said it is considering testing all pregnant women for Zika.

Inadequate screening by health authorities across Southeast Asia is likely to lead to significant under-reporting of the spread of Zika, regional experts say.

Thailand has confirmed 392 cases of Zika since January, with 39 pregnant women among them, while the wealthy city-state of Singapore has recorded 393 cases, including 16 pregnant women.

Despite its laissez-faire reputation among travellers, Thailand remains largely conservative, and Theravada Buddhism, the form of the religion practiced by up to 95 percent of its people, regards abortion as a sin.

That might lead some doctors to decline to terminate pregnancies, Pisek said, adding, "Buddhism won't affect the law, but some doctors might refuse."