Looks like it’s a bad time running for U.S. based financial institutions. Not much time has passed when five U.S.-based banks, U.S. Bancorp, JPMorgan Chase, Bank of America, PNC Financial Services Group, and SunTrust became a target of DDoS attacks. And now again, hacker group called Izz ad-Din al-Qassam Cyber Fighters warned it would launch a series of distributed denial of service (DDoS) attacks against these banks.

Following this warning, some of the bank customers started reporting problems accessing banking websites Tuesday evening, and some sites were still intermittently inaccessible on Wednesday afternoon.

“PNC and other banks have experienced an unusual volume of internet traffic. As a result, some customers may experience slowness or difficulty when logging into online and mobile banking. We are working to resolve this issue as quickly as possible. Please continue to follow our page for additional updates. We apologize for the inconvenience and appreciate your patience,” PNC posted on its Facebook page Tuesday evening.

Actually, it was PNC that faced the most problems due to DDoS attacks, while there were no outage reports on Sitedown.co for US Bank, JPMorganChase or Suntrust as of this time.

The hackers also posted some message on Pastebin, reinforcing their commitment of more severe attacks.

“In [the] new phase, the wideness and the number of attacks will increase explicitly; and offenders and subsequently their governmental supporters will not be able to imagine and forecast the widespread and greatness of these attacks,” the group wrote.

Over the time, DDoS attacks are getting larger and more serious. These are actually more devastating to businesses like financial institutions as have the power to take down organizations for long amounts of time, which in turn is [painful for both customers and the business itself.

In a similar incident, Anonymous hacktivist affiliate–Fawkes Security took down the HSBC websites worldwide via DDoS attacks. The hacktivist group posted the information on Pastebin.com.