Insight: A new wave of U.S. mortgage trouble threatens

The loans are a problem now because an increasing number are hitting their 10-year anniversary, at which point borrowers usually must start paying down the principal on the loans as well as the interest they had been paying all along.

... the worst case scenario for some banks could be bad, eating deeply into their earnings and potentially cutting into their equity levels at a time when banks are under pressure to boost capital levels.

Banks have some options for reducing their losses. They can encourage borrowers to sign up for a workout program if they will not be able to make their payments. In some cases, they can change the terms of the lines of credit to allow borrowers to pay only interest on their loans for a longer period, or to take longer to repay principal.

A Bank of America spokesman said in a statement that the bank is reaching out to customers more than a year before they have to start repaying principal on their loans, to explain options for refinancing or modifying their loans.