The U.S. Bank spokesman said the issue appeared to be related to problems at other banks in the last week.
Senate Homeland Security Committee Chairman Joe Lieberman (Ind., Conn.) has said Iran is targeting the American financial system for cyberattacks in response to U.S. sanctions placed on the country because of its nuclear program.
The Quds Force, a secretive Iran military unit blamed for terrorist activity, probably executed the cyberattacks, he said last week.
"I think this was done by Iran and the Quds Force, which has its own developing cyberattack capacity," he continued.
"Iran has not hacked the U.S. banks," Gholam Reza Jalali, head of Iran's Civil Defense Organization, told the semiofficial Fars News Agency on Sunday.
Lieberman was observing Yom Kippur on Wednesday and was not available for comment.
An attack was directed at Wells Fargo & Co. on Tuesday. The so-called denial of service attacks, which flood a website with communications requests, caused disruptions in the banks' online operations.
Outages also have hit Bank of America Corp., Citigroup Inc., and JPMorgan Chase & Co.
Lieberman has been pushing legislation to try to beef up the network defenses of privately owned entities such as financial networks, electric utilities, and chemical plants amid warnings of vulnerabilities by top U.S. officials.
Business groups have opposed the new standards, and last month, the Senate failed to pass a watered-down version of the bill.
The recent cyberattacks on banks are "a powerful example of our vulnerability," Lieberman said.