The nation's deposit insurance fund declined 24 percent in the first three months of the year, to $13 billion, as the number of banks failing mounted, the Federal Deposit Insurance Corp. said today.

The agency, which ensures that depositors do not lose their money up to a limit if a bank fails, reported 21 banks failing in the first quarter. And the agency has taken further hits since the close of the first quarter on March 31. Last week, Florida-based Bank United collapsed, costing the FDIC $4.9 billion.

Overall, 36 banks have failed this year.

The number of "problem" banks -- those at risk of failing -- increased in the quarter from 252 to 305. The agency doesn't name these banks.

FDIC Chairman Sheila Bair said she expects the fund to continue to decline through the year, but stay positive. The agency has already set aside $28 billion from its deposit insurance fund to cover losses over the next year. And it is expecting to raise $8 billion in fees from the banking industry through June 30, including a special $5.6 billion assessment approved last week.