The Federal Deposit Insurance Corp. is becoming concerned about mounting unrealized losses in U.S. banks' bond portfolios and the possibility that those losses will have to be realized, according to the regulator's acting chairman.

"Right now, our banks have strong liquidity, so they shouldn't have to dispose of those assets," Martin Gruenberg said at a Nov. 30 Senate Banking Committee hearing. "But as the market evolves and banks may have to dispose of those assets, there are substantial unrealized losses that could impact our institutions."

Gruenberg, who has been nominated for the permanent chair position, labeled the increase in unrealized losses as a "very substantial" overhang for banks that could soon become "problematic."

As the Federal Reserve has rapidly raised interest rates over the last year, the value of many bonds that banks own have dropped. That has led to significant unrealized losses in banks' available-for-sale securities portfolios, which have taken a chunk out of accumulated other comprehensive income, or AOCI, and driven down tangible book values.

The U.S. banking industry recorded total AOCI losses of $423.58 billion during the first six months of the year, according to an August analysis by S&P Global Market Intelligence. A more recent analysis found that the 15 largest U.S. banks' AOCI losses tumbled again in the third quarter, falling $33.80 billion sequentially.

Gruenberg said banks' unrealized losses grew even larger in the third quarter.