Wells Fargo's website was experiencing problems Wednesday, after a threat against US banking firms from a group pledging retaliation for an online video which has sparked unrest in the Muslim world.
Wells Fargo said in a Twitter message late Tuesday that its customers "may be experiencing limited access" to the bank's site, but offered no further details.
The problems arose hours after a group calling itself "Cyber fighters of Izz ad-din Al qassam" announced that Wells Fargo and other banks would be targeted for the second week of "Operation Ababil."
The Site Intelligence Group, which monitors threats from the so-called "global jihadist network," said the group had posted a message warning against attacks on Wells Fargo on Tuesday, US Bank on Wednesday and PNC Bank on Thursday.
A similar message was posted last week to protest the controversial "Innocence of Muslims" video on YouTube.
The group said it launched attacks last week under the name "Operation Ababil," meaning "Swarm," against the websites of Bank of America and the New York Stock Exchange on September 18, and Chase on September 19.
The two banks last week acknowledged problems with their websites but did not elaborate on the reasons.
A US financial industry group warned banks and other institutions to brace for potential cyber attacks following last week's message.
The Financial Services Information Sharing and Analysis Center said it raised its cyber threat level from "elevated" to "high."
The group, which monitors cyber threats to the sector, cited "recent credible intelligence regarding the potential for DDoS and other cyber attacks against financial institutions."