Goldman Sachs Group Inc. and Citigroup Inc. stepped up warnings to shareholders about cyberattacks as the federal government prodded banks and government agencies to bolster their defenses, Bloomberg reported.

Online and mobile banking give new points of entry that can be used to disrupt or penetrate operations, the two New York-based firms said in annual regulatory filings. The companies said they're vulnerable to tactics that overload websites to shut off public access, such as attacks that disrupted the nation's largest lenders late last year.

U.S. banks speculate that foreign nations, organized crime or terrorists are behind efforts to cripple their websites and warn that costs to keep intruders at bay will rise. Read more.