President Obama in February requested an emergency appropriation of $1.8 billion to deal with Zika, but the Republican-led Congress said the administration should first use money set aside to combat Ebola. After arguing for months that that was not possible, officials said last week that they would use $510 million of that money, plus $79 million from other accounts.

But Dr. Fauci and Dr. Anne Schuchat, the deputy director of the Centers for Disease Control and Prevention, argued on Monday that such a stopgap measure was not enough. If Congress does not provide the needed funds, Dr. Fauci said, public health authorities will probably have to divert money from malaria and tuberculosis prevention programs, as well as from flu vaccine programs.

The mosquito that carries the Zika virus is present in 30 states, more than twice what officials originally thought, Dr. Schuchat said, though no locally acquired cases have been reported yet. That indicates that mosquitoes in the states do not yet have the virus. But in Puerto Rico, nearly every confirmed case has been contracted locally, meaning the virus is present in mosquitoes there and transmitting rapidly.