The latest in what appears to be a coordinated series of cyber attacks hit U.S. Bancorp's public website Wednesday morning.
Earlier in the day, Wells Fargo acknowledged in a statement that some customers were difficulty getting into their online accounts.
A spokesman for the Minneapolis-based bank said that while its site, USbank.com, hasn't completely gone down there have been "intermittent connectivity issues" that began about 8:30 a.m. and were continuing at midday.
Pages have been slow to load and some customers can't access the site at all, said bank spokesman Tom Joyce.
The bank's IT staff is working to fix the problem, he said. Joyce described the disruptions as a "denial of service" attack but said the bank has no idea who is behind it.
Denial of service attacks are something like an online flash mob aimed at disrupting websites with a lot of traffic.
"This is all related to unusual and coordinated high traffic volume designed to slow the system down," Joyce said in an interview. "It's very similar to what other large banks have experienced in the past week.
"We can assure customers that their data and their funds are secure," he said. Joyce said the bank is working closely with federal law enforcement officials on the matter.