PNC Bank is apologizing to customers in an email for any problems they are having accessing their accounts online, saying that actions it has been taking to combat a cyber attack may have blocked "a small percentage" of customers from using the bank's website.

Customers of the Pittsburgh-based bank began reporting unusual problems logging into their accounts Dec. 11, after a protest group threatened a round of denial-of-service attacks against several big banks. The attacks work by flooding websites with communications requests.

"PNC has taken steps to block this traffic, and maintain online and mobile banking access for the vast majority of its customers," the bank said in the customer email sent Thursday night.

In some cases, those countermeasures "also may have blocked access to a small percentage of legitimate PNC customers for an extended period."

"We sincerely apologize to those affected," the email said.

The bank urged customers locked out to keep trying, or to call toll-free 1-888-762-2265 for help.

"Please be aware that given the current situation, you may experience longer than usual wait times," the email said.

The bank added that customer accounts were secure and that any transfers or online bill payments already scheduled would continue to be processed even for customers having access problems.

The bank did not give any timetable for restoring full access.

First Published: January 5, 2013, 5:00 a.m.