Citigroup (NYSE: C) is seeing its website slow down amid a wave of distributed denial of service attacks on the nation's biggest banks.

Some customers of the third-largest U.S. bank by assets reported difficulty accessing their accounts on Wednesday, a day after a hacktivist group that has claimed responsibility for a series of electronic assaults on at least 10 banks since September threatened more attacks.

"This afternoon, Citi experienced temporary interruptions in the availability of some of its websites," Citigroup spokesman Andrew Brent said Wednesday night in an email. "We worked to resolve the situation in a matter of hours and continue to monitor online activity."

Visitors to, a website that tracks website outages, recorded 144 reports Wednesday night from people who claimed to have difficulty logging in to their accounts at Citigroup within the most recent 24 hours, compared with a total of 154 reports in the past week.

The slowdown follows a vow Tuesday by the al Qassam Cyber Fighters Group to sustain the assaults, which can overwhelm banks' websites and prevent customers from accessing their accounts. "This week also widespread attacks will be carried out on U.S. banks like previous weeks," the group wrote in a message on Pastebin, a website used by programmers.

The group pledges to continue the barrages as long as a trailer for an anti-Muslim film remains on YouTube, which says the trailer complies with the company's content guidelines.

BB&T (BBT), Wells Fargo (WFC), U.S. Bank (USB) and PNC Bank (PNC) all have seen their websites besieged recently as well.