WASHINGTON — Top health officials in the United States said evidence of the Zika virus found in two Brazilian babies with microcephaly is the strongest connection yet between the disease and the birth defect.

Centers for Disease Control and Prevention Director Dr. Thomas Frieden spoke before two subcommittees at a joint hearing of the House Foreign Affairs Committee on Wednesday about a recent study conducted in Brazil on infants with microcephaly who died in their first 24 hours. Researchers found the Zika virus in the tissue of those babies.

 “Working with our Brazilian colleagues, the CDC laboratory was able to identify the genetic material of the Zika virus in the brain tissues of the two infants who died of microcephaly,” he said. “This is the strongest evidence to date that Zika is the cause of microcephaly.”

Friedan called on Congress to accept President Barack Obama‘s budget proposal to provide more than $1.8 billion in emergency fundings for Zika care and research.

Still more research needs to be done to establish a causal connection between Zika and microcephaly. Of the proposed $1.8 billion in emergency funding, $828 million would go to the CDC, with a chunk going toward “understanding the clinical and epidemiological patterns to make it definitive,” Frieden said.

Zika is carried by mosquitos. It causes rashes and fevers, though one in five people show no symptoms. There have been reports of women infected with the disease during pregnancy delivering babies with microcephaly.

There is no treatment for Zika, and a vaccine is a few years away, but the National Institutes of Health is developing vaccines and could start Phase 1 trials by the middle of summer, said Dr. Anthony Fauci, director of the NIH’s allergy and infectious diseases institute.

“While these approaches are promising, it is important to realize that the development of investigational vaccines to establish whether they are safe and effective takes time,” Fauci told the panels.

As of Tuesday, more than 30 countries and territories reported local transmissions of the Zika virus, Frieden said. Eleven states, Washington, D.C., and two U.S. territories have reported laboratory-confirmed cases.

For that reason, the CDC put its emergency operations center on “Level 1” status Monday, the highest possible alert level. It has activated Level 1 status three times in its history: during the Ebola crisis in 2014, the H1N1 or swine flu outbreak in 2009 and after Hurricane Katrina in 2005.

In the meantime pregnant women in any trimester and women who are trying to get pregnant should avoid traveling to countries affected by Zika, the CDC warns.

The CDC said it is not aware any other virus that has caused a significant number of birth defects in the past 50 years, Frieden said.