The Centers for Disease Control and Prevention is warning pregnant women to stay away from 11 Southeast Asian countries where Zika is spreading — including Thailand, where officials on Friday reported the first confirmed cases of birth defects linked to the virus.

A city worker sprays chemicals with a fumigator to kill mosquitoes in an effort to control the spread of the Zika virus at a school in Bangkok. LILLIAN SUWANRUMPHA / AFP - Getty Images

“Pregnant women should not travel to any area with a Zika travel notice and should consider postponing non-essential travel to the 11 countries in Southeast Asia listed in the newly issued considerations,” the CDC advised on Thursday.

"We have found two cases of small heads linked to Zika, the first cases in Thailand."

“The countries included in these considerations are Brunei, Burma (Myanmar), Cambodia, Indonesia, Laos, Malaysia, Maldives, Philippines, Thailand, Timor-Leste (East Timor), and Vietnam.”

Thailand confirmed Friday it had "found two cases of small heads linked to Zika," Prasert Thongcharoen, an adviser to Thailand’s Department of Disease Control, was quoted by Reuters as saying.

The CDC also updated its recommendations to women considering becoming pregnant and men whose partners were considering it. Both women and men should consider just staying away from Zika zones if they're planning a pregnancy, the CDC said, and it said men who have been in areas where Zika is spreading should avoid unprotected sex for six months, even if they haven't had symptoms of infection.

The Zika strain spreading in Southeast Asia is somewhat different from the one spreading across the Americas. It’s not clear if there are new outbreaks in that region, or whether doctors are finding it in more patients because health officials are on the alert and more patients are being tested.

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Either way, it’s worth taking precautions, the CDC says.

Zika is linked to severe birth defects in babies born to women who became infected while pregnant. Doctors say they’re finding almost daily evidence of new defects caused by Zika in babies — from the well-publicized small head called microcephaly to less visible damage to the brain, eyes and limbs.

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The CDC has reported thousands of confirmed Zika cases that U.S. travelers have brought back to the states with them, including more than 800 pregnant women. CDC experts say there are likely thousands more that have not been reported.

Florida reports 134 home-grown cases — presumably caused by travelers infected in other Zika zones who were then bitten by local mosquitoes. Southern states, including Florida, are home to the Aedes aegypti mosquito that spreads Zika.

“Zika virus has been present in areas of Southeast Asia for many years, and several countries have reported occasional cases or small outbreaks. Recent variations in the number of cases reported in the area have been observed,” the CDC said.

"The countries included in these considerations are Brunei, Burma (Myanmar), Cambodia, Indonesia, Laos, Malaysia, Maldives, Philippines, Thailand, Timor-Leste (East Timor), and Vietnam.”

“Zika virus is considered endemic in some of these countries, and many people who live there are likely immune. But U.S. travelers to areas where Zika is endemic may not be immune to the virus and infections have occurred in travelers to Southeast Asia.”

The CDC says it’s not clear how big the risk actually is. Zika doesn’t cause symptoms in most people who get it, so it can spread quietly.

“Travelers to areas where Zika is actively spreading who have not already been infected with Zika virus can get it from mosquito bites,” the CDC said.

“Some travelers to areas with Zika will become infected while traveling but will not become sick until they return home and they might not have any symptoms. To help stop the spread of Zika, travelers should use insect repellent for three weeks after travel to areas with Zika to prevent mosquito bites.”

Zika can also spread sexually, so men and women alike who travel to Zika-affected areas should take precautions to avoid pregnancy. And pregnant women need to avoid having unprotected sex with anyone who’s traveled recently to an affected area.

“If you or your partner live in or travel to these areas, use condoms from start to finish every time you have sex or do not have sex during the pregnancy. Sex includes vaginal, anal, and oral sex,” the CDC advised.

Zika was never linked with birth defects until Brazil noticed an upsurge in microcephaly cases last year that coincided with a new Zika outbreak. Scientists are trying to figure out if the birth defects had been there all along and simply not linked to Zika, or whether something new is going on.

Until recently, there had not been a good test that could tell the difference between someone infected with Zika, dengue virus or chikungunya virus, all of which circulate in many of the same areas.

The Zika strain spreading in Southeast Asia is somewhat different from the one spreading across the Americas. It’s not clear if there are new outbreaks in that region, or whether doctors are finding it because they are looking for it for the first time.

Either way, it’s worth taking precautions, the CDC says.