Congress needs to approve a contentious request for $1.9 billion in emergency mandatory funding to combat the Zika virus before the summer hits, a leading administration official said.

Zika is primarily spread through mosquito bites, and health officials are worried about the virus spreading particularly in warmer southern states. The Obama administration asked for nearly $2 billion in emergency funds, but Congress has been skeptical.

"The funding needs to come and is time sensitive," Health and Human Services Secretary Sylvia Mathews Burwell said during a Senate appropriations subcommittee hearing.

Obama's request has been met with skepticism from Republicans who say there is more than $1 billion left over from fighting the Ebola outbreak, which has waned.

Burwell said that money is already committed to countries hit hard by Ebola, primarily those in West Africa.

If countries meet certain goals, HHS will dole out money to them, Burwell said.

She added cases of Ebola are still popping up. Burwell pointed to Sierra Leone, one of the countries hit hard by the outbreak that killed more than 10,000 people.

After the country was declared Ebola-free, "four days later we found another case on a dead body, because we were still swabbing and testing the dead bodies," Burwell said.

Zika has spread to more than 30 countries and territories, primarily in central and South America. The U.S. has had more than 150 cases, but so far the virus hasn't spread through mosquitoes.

Almost all of the cases are people returning from a country where Zika is spreading by mosquitoes. The other cases are through sexual transmission.

Burwell said the mosquito that spreads Zika can bite up to four people a day and can breed easily. The money would go to research the link between Zika and a birth defect called microcephaly that causes brain damage, and to help control the mosquito populations.