James Dimon acknowledged on Wednesday that he knew about trading losses at one of JP Morgan Chase's units when he dismissed concerns over its outsize bets as "a complete tempest in a teapot" in April. However, Dimon told the Senate banking committee that senior executives had assured him the problems were an aberration.

The 56-year-old chief executive of the nation's largest bank by assets told senators that "some issues" had come to his attention before April 13, when he made the dismissive comments on a conference call with analysts. Trading losses developed after The Wall Street Journal reported on April 6 that a trader known as "the London Whale" had roiled a sector of the debt markets with his large bets, said a person close to the bank.