Top federal health officials want Congress to know they urgently need more money to fight the Zika virus — and that time is running out.

In media interviews and briefings on Thursday, Centers for Disease Control and Prevention (CDC) Director Tom Frieden stressed that funding from Congress is “crucially important and urgently needed.”

{mosads}He repeatedly warned the current funding levels would leave pregnant women worldwide vulnerable to the virus, which has been linked to debilitating birth defects.

Without more funding, he said vaccines would be developed more slowly, mosquitoes would spread just as rapidly and more babies could be born with microcephaly, a condition that can total $10 million in healthcare costs per child.

“We are scraping together every dime we can. It’s not easy to do that, and it makes the response much more complex and much less smooth,” Frieden, who normally shies away from politics during his briefings, told reporters Thursday.

“It’s worth trying whatever might work to protect women,” Frieden said. 

Dr. Anthony Fauci, the top infectious disease expert at the National Institutes of Health, warned that the development of at least three vaccines would have to slow down or stop without the funding.

“If we don’t get the money that the president asked for, it’s going to slow down a number of things, not just vaccines,” Fauci said.

Frieden launched his funding push a day after returning from a three-day trip to Puerto Rico, which has seen hundreds of cases of Zika. He warned that the island’s rainy season is just ahead and that “hundreds of thousands” of people are likely to be infected by the end of 2016.

Congressional Republicans, though, are firm in their opposition to the new funding, with many arguing that the White House should use funding left over from its fight against the 2014 Ebola epidemic in Africa. 

The head of the House Appropriations Committee, Rep. Hal Rogers (R-Ky.), said Thursday the “necessary funding to address the crisis in the near term is already available.” Frieden and Fauci have both protested that claim, arguing that Congress has already set aside those funds to build up the nation’s defenses against other epidemics.

Energy and Commerce Committee Chairman Fred Upton (R-Mich.), who earlier on Thursday posted an op-ed warning about the Zika virus’s spread, supports a response to Zika, according to a spokeswoman.

Upton is reviewing whether there are any current funding streams that could be used for an immediate response instead of relying on the supplemental request from the White House. 

Frieden, who is based in Atlanta, will come to D.C. next week to continue making the case for the funding request in Congress. The money would help prevent, treat and diagnose the Zika virus, which is expected to spread in the U.S. this spring.