The fourth-largest U.S. bank by assets confirmed that some of its customers experienced intermittent access issues on Tuesday. The bank was working to quickly resolve the issues, a spokeswoman said, declining to comment on the source of the problem.

A financial services industry group last week warned U.S. banks, brokerages and insurers to be on heightened alert for cyber attacks after Bank of America Corp and JPMorgan Chase & Co experienced unexplained outages on their public websites.

Wells Fargo's problems came the same day that an unidentified person on the Internet called on "cyberspace workers" to attack the bank's site. In a posting on pastebin.com, the person also warned of attacks later this week against U.S. Bancorp and PNC Financial Services Group Inc.

A similar posting last week made threats against Bank of America and the New York Stock Exchange. The poster said the attacks will continue until the film that had stirred up anti-U.S. protests across the Middle East was removed from the Internet.

A U.S. Bancorp spokesman said the bank was aware of the posting and was working closely with law enforcement authorities. A PNC spokesperson did not immediately respond to a request for comment.

Senator Joseph Lieberman, chairman of the Senate's Homeland Security and Governmental Affairs Committee, said on Friday that he believes Iran was behind the attacks.

Reuters reported on Friday that Iranian hackers have repeatedly attacked Bank of America, JPMorgan and Citigroup Inc over the past year as part of a broad cyber campaign targeting the United States, according to people familiar with the situation.

The attacks, which began in late 2011 and escalated this year, have primarily been "denial of service" campaigns that disrupted the banks' websites and corporate networks by overwhelming them with incoming web traffic, said the sources.

(Reporting By Rick Rothacker in Charlotte, North Carolina; Editing by Jacqueline Wong)

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