Two newborns in California have a severe birth defect caused by the Zika virus, the state's public health department announced Thursday.

The babies have a condition known as microcephaly, where the brain is underdeveloped, leading to abnormally small heads and severe developmental problems. Their mothers had been infected with Zika while staying in countries where the virus is spreading broadly.

While 13 newborns in the U.S. have been reported as having microcephaly, the two babies born in California are the first in that state. California has reported 114 Zika cases, all of them travel-related. State health officials said Thursday there is "no evidence" that mosquitoes are transmitting the virus in California currently.

The California Department of Public Health would not release details of the mothers or where they had traveled, saying only that one of them has returned to her home country, where the Zika virus is being actively transmitted.

Public health officials have been warning Americans to guard against mosquitoes and use protection during sex with someone who has traveled to a Zika-affected country, as the virus can be spread sexually. The Centers for Disease Control and Prevention also has warned pregnant women not to travel to an area north of downtown Miami, where the virus is spreading through mosquito bites.

"This is a sobering reminder for Californians that Zika can cause serious harm to a developing fetus," Karen Smith, director of California's Department of Public Health.