JP Morgan Chase, the largest bank in the United States, acknowledged on Thursday a surprising trading losses of 2 billion U.S. dollars for the past six weeks.

The losses came from derivatives bets which have gone wrong in the bank's Chief Investment Office, a unit that manages risk for the New York-based bank.

After factoring in other securities gains, JP Morgan raised its estimates for Chief Investment Office's net losses to 800 million dollars in the second quarter. The unit was previously expected to gain 200 million dollars.

Jamie Dimon, chief executive of JP Morgan, told reporters in a surprise conference call after U.S. markets closed that the losses were caused by "errors, sloppiness and bad judgement."

"This was a unique thing we did," Dimon said. "Obviously it had a lot of problems. It was a bad strategy. It became more complex, it was poorly managed."

The bank's stocks plunged nearly 7 percent in after-hours trading after the losses were announced. Other bank stocks, including Citigroup, Goldman Sachs and Bank of America also suffered heavy losses.