The Obama administration is warning Congress that it will run out of money by the end of the month to develop a Zika vaccine, jeopardizing future clinical trials.

Health and Human Services Secretary Sylvia Mathews Burwell wrote in a letter to congressional appropriators Wednesday that the National Institutes of Health will exhaust its money for Zika vaccine development and that a second phase of clinical trials will be delayed. NIH started a preliminary clinical trial on Wednesday.

"A delay in this stage of development will delay when a safe and effective Zika vaccine is available to the American public," Burwell wrote. "In addition, research and development of other vaccine candidates, diagnostics, therapeutics and vector control technologies may be constrained."

NIH officials have long said that future clinical trials could be jeopardized if funding isn't approved, but a clear timeline on when it runs out of funds hasn't been given before.

The administration had hoped to start a larger clinical trial early next year to test the vaccine's effectiveness, but that trial may be delayed.

Florida health officials have found 15 cases in Miami through mosquito bites, the primary mode of transmission.

Currently there is no vaccine or treatment for Zika, which normally causes a mild illness but can cause a birth defect called microcephaly.

The letter comes as Congress squabbles over Zika funding.

Congress adjourned last month after a $1.1 billion funding package passed the House but stalled in the Senate due to Democratic objections to several riders. The bill was less than the $1.9 billion President Obama requested in February.

After the first mosquito-borne transmissions of Zika emerged on Friday, Democrats called for Congress to return from recess early and pass the Zika funding measure.

Senate Majority Leader Mitch McConnell's office said that he is open to consider the bill again as long as Democrats drop their objections to the $1.1 billion package.