The FBI is working with the Secret Service to determine the extent of cyber attacks against several US financial institutions.
A report on Bloomberg.com said the FBI was investigating an incident in which Russian hackers attacked the American financial system this month in possible retaliation against US government-sponsored sanctions aimed at Russia.
The attack, Bloomberg said, led to the loss of sensitive data. Bloomberg cited security experts saying that the attack appeared "far beyond the capability of ordinary criminal hackers".
The New York Times, citing people familiar with the matter, said JPMorgan Chase, the largest US bank by assets, and at least four other firms, were hit this month by co-ordinated attacks that siphoned off huge amounts of data, including cheque and savings account information.
JPMorgan spokeswoman Trish Wexler did not confirm the reports but said large companies experienced cyber attacks nearly every day.
"We have multiple layers of defence to counteract any threats and constantly monitor fraud levels," she said.
The Wall Street Journal cited unnamed sources in a report that called the attacks a "significant breach of corporate computer security".
The FBI said combating cyberthreats and criminals remained a top priority for the US government and it was "constantly working with American companies to fight cyber attacks".