WASHINGTON (Reuters) - Two senior Obama administration officials wrote congressional leaders on Tuesday to urge legislation be passed to fund the fight against the Zika virus, as concerns mount the United States will soon face locally transmitted cases of the disease.

U.S. health officials concluded this month that the virus, spreading rapidly in the Americas, was a cause of microcephaly, a rare birth defect defined by unusually small head size that can hamper development. Zika is also linked to other health problems.

The Obama administration asked the Republican-controlled Congress on Feb. 22 for $1.9 billion in emergency supplemental funding to address the health threat from Zika.

With Congress failing to move quickly on that request, the administration earlier this month redirected $589 million to prepare for Zika, mostly from a fund to fight the Ebola virus.

"Without significant additional appropriations ... the nation's efforts to comprehensively respond to the disease will be severely undermined," Susan Rice, President Barack Obama's national security adviser, and Shaun Donovan, director of the Office of Management and Budget, wrote in the letter.

It was sent to Senators Mitch McConnell and Harry Reid, the top Republican and Democrat in the Senate, and other congressional leaders. It can be seen at https://www.whitehouse.gov/sites/default/files/zika_letter_4.26_mcconnell.pdf

The mosquito that carries the virus, Aedes aegypti, is found in 30 states, and health officials worry that as temperatures warm, the virus will move north from countries including Brazil and cause locally transmitted cases. Nearly 390 travel-related Zika cases have been reported in the United States and 500 locally acquired cases in U.S. territories.

Last week, a bipartisan group of U.S. senators said they would soon offer a proposal to an appropriations measure for more funding.

Rice and Donovan said in the letter they were pleased to hear of that support but were "concerned about the adequacy and speed of this response."

White House spokesman Josh Earnest said in a press briefing that lawmakers were talking about "barely half of what we have indicated is needed." The White House was also worried that funding being discussed on Capitol Hill "is tied to a moribund appropriations process," Earnest said.

(Reporting by Timothy Gardner; Editing by Peter Cooney)