For example, after Singapore's Zika outbreak in late August, the CDC issued a travel alert and urged pregnant women to avoid traveling there. Singapore has reported nearly 400 cases of locally transmitted Zika.
The newly issued travel considerations come as countries elsewhere in the region have reported an increase in the number of cases. Thailand is also investigating cases of microcephaly in newborns to determine whether they may be linked to Zika infection.
Unlike Brazil and many parts of the Americas that have been hit hardest by the recent Zika epidemic, areas of Southeast Asia have had the Zika virus present for many years. It is considered endemic in these countries, the CDC said in a statement Thursday, and many people who live there are probably immune.
But U.S. travelers to areas where Zika is endemic may not be immune to the virus, the CDC said, and infections have occurred among travelers to Southeast Asia.
"The level of risk for Zika virus infection in these countries is unknown, but it is likely lower (but not zero) than in areas where Zika is newly introduced and spreading widely," the agency said.
Because Zika virus infection during pregnancy causes severe birth defects, including microcephaly and other brain abnormalities, the CDC is advising pregnant women to talk to their health-care providers and consider postponing non-urgent travel to Southeast Asia.