A spokeswoman for the No. 4 U.S. bank by assets said some customers may have intermittent access to their online banking, although the high volume of traffic that has flooded the site has declined.

"Our technical teams have been working around the clock to ensure our Web site is accessible to our customers," bank spokeswoman Bridget Braxton said. The bank has been posting apologies on its Twitter account.

Since September, a hacker activist group called the Izz ad-Din al-Qassam Cyber Fighters has said it was targeting major banks with so-called denial of service cyber attacks. These attacks can disrupt service by deluging Web sites with high traffic.

On Tuesday, the group said in an Internet posting that it would target the "5 major US banks." In a similar posting last week, it forecast attacks against banks that included PNC Financial Services Group Inc and U.S. Bancorp, which reported some disruptions.

A PNC spokesman on Friday said the bank's systems were operating normally. Spokespersons for Bank of America Corp, JPMorgan Chase & Co and U.S. Bancorp declined to comment. Citigroup Inc could not be immediately reached.

In its alert on Friday, the U.S. Office of the Comptroller of the Currency, which regulates national banks and thrifts, said groups launching denial of service attacks had varying motives, from gaining public attention to diverting the attention of banks while launching simultaneous attacks to commit fraud or steal proprietary information.

"Banks need to have a heightened sense of awareness regarding these attacks and employ appropriate resources to identify and mitigate the associated risks," the alert said.

Banks should have sufficient staffing during attacks, work with third-party providers and share information with other banks, the OCC said.

Of five major banks, Wells Fargo on Friday had spurred the most complaints from users about access problems, according to the Web site SiteDown.co, which tracks customer reports. It listed 576 "downtime reports" in the past 24 hours.

Wells Fargo says it has 21 million active online banking customers.

(Reporting By Rick Rothacker in Charlotte, N.C.; Editing by Gerald E. McCormick)

• A Permanent Economic Emergency