WASHINGTON — U.S. banks will prepay about $45 billion in premiums to replenish a federal deposit insurance fund now in the red, under a plan adopted Thursday by federal regulators.

The Federal Deposit Insurance Corp. board voted to mandate the early payments of premiums for 2010 through 2012. Amid the struggling economy and rising loan defaults, 120 banks have failed so far this year, costing the insurance fund more than $28 billion.

To address concerns of small banks in weak financial condition, the FDIC also set up an exemption process for those that prove the prepaid fees would be a hardship.

The FDIC expects the cost of bank failures to grow to about $100 billion over the next four years.

It is the first time the agency has required prepaid insurance fees. The idea behind it is for banks to spread the costs over three years rather than paying a one-time fee that would deplete their capital reserves.

Unlike a one-time fee, the prepaid premiums won't affect banks' earnings "during these difficult times," FDIC Chairman Sheila Bair said before the vote.

The new premiums were proposed by the FDIC in late September and opened to public comment. They come atop a special emergency fee that took effect at midyear, estimated to have brought in about $5.6 billion.

The early payment will be "manageable" for Southern Arizona Community Bank in Tucson, said its president and CEO John P. Lewis. His institution has been paying an average of around $25,000 per quarter for regular insurance premiums, plus about $70,000 this year for the special fee.

The $70,000 "is a little rough on a community bank," Lewis said in a telephone interview. "It would push some banks over the edge."

The three-year prepay likely will cost Southern Arizona Community roughly $300,000 but will work out under the plan to about $9,000 a month, he said — against a cash balance now at $9.5 million.

Untouched will be the bank's net profits expected at around $500,000 this year and $750,000 in 2010.

Daniel Blanton, president and CEO of Southeastern Bank Financial Corp. and Georgia Bank & Trust of Augusta, said his bank's assessments doubled from 2008 to 2009, and the FDIC's special fee this year made the number even higher. Compared with those costs, he said, the impact of prepayment is minimal.

"The cost is just the earning capacity of that money I paid in advance — and in this (low interest rate) environment that's not very dramatic," Blanton said by phone.