The Office of the Comptroller of the Currency told financial institutions on Friday to report cyber attacks to law enforcement and alert customers to their impact as new assaults targeted PNC Financial Services Group Inc. and other banks.

The alert warned about a wave of so-called distributed denial-of-service attacks. Such actions harness networks of infected computers to pump large volumes of Internet traffic at websites, often causing slowdowns or disruptions.

Attacks starting last week have hit PNC, Bank of America Corp., JPMorgan Chase & Co., U.S. Bancorp, Wells Fargo & Co. and SunTrust Banks Inc., according to two executives at companies providing security to some of the targeted banks. They asked not to be named because they weren't authorized to discuss clients and didn't want their companies to become targets of computer assaults.

A group calling itself Izz ad-Din al-Qassam Cyber Fighters announced plans to attack banks in a Dec. 10 statement posted on the website pastebin.com. The same group claimed responsibility for a series of attacks against financial institutions in September and October, saying they were in response to a video uploaded to Google Inc.'s YouTube ridiculing the Prophet Muhammad and offending some Muslims.

The comptroller's office, which didn't identify targeted banks or the groups responsible for the attacks, said fraudsters can use them to distract bank personnel to gain access to customers' accounts, block customers from reporting suspected fraud and prevent banks from alerting customers.

The current attacks use similar methods as in the earlier assault, including targeting firewalls or intrusion-detection systems, said Carl Herberger, a vice president at Radware Ltd., a Tel Aviv-based network security provider working with some of the banks.

Because the attackers can shift tactics and targets, banks should share information with each other and give "timely and accurate" advisories to customers on website problems and precautions they can take, the comptroller's office said.

PNC, based in Pittsburgh, said in a statement posted on its website that it's aware of a "potential" cyber threat that could make it difficult for customers to log into their accounts.

"Please be assured that PNC's website is protected by sophisticated encryption strategies that shield customer information and accounts," the statement reads. "We have no information regarding timing, duration or intensity of this potential threat."

Wells Fargo, based in San Francisco, said in a statement Dec. 19 that its website was experiencing an "unusually high volume of traffic" and said the vast majority of its customers weren't affected. The bank encouraged customers that experience problems to access accounts through its stores and automated teller machines, or by phone.