PNC Bank's efforts to combat the recent attacks on its website may be preventing legitimate customers from accessing their accounts online.

The bank has sent its customers an email apologizing for disruptions caused both by the ongoing cyberattacks on its site and the bank's own measures to block those attacks. The email suggests that inconvenienced customers should use other banking channels, such as a local branch, ATM, or phone banking.

The email also notes that the bank's website has been hit with "unusually high volume of traffic" that is "consistent with threatened cyber attacks on the U.S. banking system," and that PNC has been taking steps to block these attacks and maintain access to the site for its customers.

These measures "may also have blocked access to a small percentage of legitimate PNC customers for an extended period," the email says.  A PNC spokesman said that "the vast majority of our customers largely have had uninterrupted access to our online banking system," and that the bank is working one-on-one with customers that have been prevented from accessing the site.

PNC Bank has been fending off cyberattacks for months. PNC is one of several U.S. banks that the hacktivist group the Izz ad-Din al-Qassam Cyber Fighters have apparently targeted in anger over an incendiary video posted online.

"Rulers and officials of American banks must expect our massive attacks! From now on, none of the U.S. banks will be safe from our attacks," the group declared in a message posted online on January 1.

The email from Pittsburgh-based PNC reassured its customers that the website is protected by sophisticated encryption techniques to safeguard their accounts and personal data.