U.S. Bank spokesman Tom Joyce said some customers experienced intermittent delays. He said the bank was working to fix the problem and was working with law enforcement.
A spokesman for PNC, Fred Solomon, said its online operations were mostly normal but some customers had trouble accessing PNC.com on their first try. He said the bank was aware of a similar threat for Thursday and was taking precautions.
The U.S. Bank spokesman said the issue appeared to be related to problems at other banks in the past week. JPMorgan Chase and Bank of America both had site problems last week, and Wells Fargo reported access problems with its site Tuesday.
Sen. Joe Lieberman, I-Conn., told C-SPAN last week he believed Iran was behind the Chase and Bank of America attacks.
Also last week, the Financial Services Information Sharing and Analysis Center, an industry group, raised its cyber threat level to "high" from "elevated" because of potential cyberattacks.
U.S. Bank is part of U.S. Bancorp, the nation's No. 8 financial company by assets, according to the most recent federal figures. PNC is part of PNC Financial Services Group, which is No. 11.