Wells Fargo on Tuesday said its online banking website was experiencing an unusually high volume of traffic that it believes stems from a denial-of-service cyber attack.

"The vast majority of customers are not impacted and customer information remains safe," said Bridget Braxton, a spokeswoman for the fourth-largest U.S. bank by assets. Customers who have trouble should try logging in again because the disruption is usually intermittent, she said.

Since September, a hacker activist group called the Izz ad-Din al-Qassam Cyber Fighters has said it was launching denial of service attacks against major U.S. banks. These attacks can disrupt service by deluging websites with high traffic.

In a posting Tuesday on pastebin.com, the group listed Wells Fargo as one of the banks "being chosen as a target." In December, Wells customers had trouble accessing the website for four days.

In its annual report filing last month, San Francisco-based Wells said it had not experienced any "material losses" related to cyber attacks but that enhancing its protections remained a priority.