Faced with new global regulations requiring them to strengthen their capital, big lenders in the US and Europe have turned to a trading tactic that flatters their positions without actually raising extra funds.

Banks that have done such "capital-relief trades" include some of the largest in the world: Citigroup, Bank of America, Deutsche Bank and Standard Chartered. But the Office of Financial Research, a US Treasury office created to identify financial-market risks, is suggesting the trades run the risk of "obscuring" whether a bank has adequate capital and pose other "financial stability concerns".