The top U.S. health officials leading the response to the mosquito-borne Zika virus sweeping through the hemisphere said its growing links to a broad array of birth defects and neurological disorders are worse than they originally suspected, increasing the risk for devastating harm during pregnancy.

Until Zika, “there has never been a mosquito-borne virus that could cause serious defects on a such a large scale,” Thomas Frieden, director of the Centers for Disease Control and Prevention, said Thursday during a conference call with reporters.

Puerto Rico is a Zika target that has Frieden “very concerned.” He expects hundreds of thousands of people there will be infected by year’s end, including thousands of pregnant women. The U.S. territory is on the “front lines of the battle,” he said.

Moreover, the latest research findings underscore the rising number of unanswered and very disturbing questions, said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

“As the weeks and months go by, we learn more and more about how much we don’t know, and the more we learn the worse things seem to get,” Fauci told reporters.

The two experts cited a study last week in the New England Journal of Medicine that examined the pregnancies of a group of Brazilian women who tested positive for Zika infection. Ultrasounds for about 29 percent of them showed fetal anomalies with “grave outcomes,” which likely means many more complications that won’t show up until after birth, Fauci said.

“That’s an alarming finding and shows the negative impact on the fetus even if the mother is infected later on in pregnancy,” he acknowledged.

Both men said U.S. efforts are underway to develop a vaccine, come up with new technologies to control mosquitoes, and monitor and protect pregnant women and their babies. But those efforts cannot be sustained over the long term without the $1.9 billion in emergency funding sought by the Obama administration. Congress has balked at approving the request.

With the approach of the rainy season in Puerto Rico and warmer temperatures in the rest of the United States, Frieden and Fauci said it is critical that the emergency funding be approved. For now, the CDC and NIH have been moving resources from existing programs. The CDC has about 750 of its staff working full-time on Zika. That includes its Dengue Branch in Puerto Rico, which has completely shifted from dengue, a related virus that is the leading cause of illness and death in the tropics and subtropics. The agency’s Fort Collins, Colo., office also has stopped working on some new tick-borne viruses to focus on Zika, Frieden said.

At the NIH, the researchers trying to accelerate a Zika vaccine are part of a team that is also developing vaccines for flu, HIV/AIDS and respiratory illness.

“There may be a point where we have to slow down at least one or maybe all three of those until we can get money,” Fauci said. “It’s a give and take. . . . You have to slow down or stop. We try very hard not to stop things.”

The cost of caring for one child with birth defects can be $1 million or more, Frieden said. In Puerto Rico, which is already facing an enormous fiscal crisis, fighting Zika is going to be an uphill battle, he noted.