Bank of America Corp. customers were unable to access its electronic banking operations and telephone call centers Friday morning.
"We're addressing the issue as quickly as possible," said Mark Pipitone, a technology and operations spokesman at the bank. "We're also working closely with our customers to help them alleviate any concerns."
The bank's ATMs were functioning normally, he said.
Pipitone declined to provide details of what caused the extensive outage.
It wasn't known if it was related to cyber attacks by a shadowy hacker group in the Middle East last September that disrupted the electronic operations at the nation's largest banks: BofA, Wells Fargo & Co., JPMorgan Chase & Co., Citigroup Inc., U.S. Bancorp and PNC Financial Services.
Those were simple denial-of-service attacks, in which a website is deluged by automated requests for service until it breaks down. But at least one other recent case involved hackers breaching bank security systems and making off with customers' funds.
Federal prosecutors last month charged a Russian, a Latvian and a Romanian with creating a computer virus that infected more than 40,000 U.S. computers in an effort to steal customers' bank-account data and other information.
The so-called Gozi virus led to the theft of unspecified millions of dollars, said U.S. Atty. Preet Bharara in Manhattan.
Customers who tried to sign on to BofA online and mobile services on Friday were greeted with a text message advising them to go to ATMs or branches.
"Our site is temporarily unavailable," it said. "We know your banking is important and appreciate your patience."
Calls to BofA's telephone banking service went unanswered as well.
At sitedown.co, an online tracker of outages at business websites, scores of customers were reporting problems. Some took potshots at the Charlotte, N.C.-based bank.
"Epic, all-points BofA outage," said one.
"Enhancing our experience?" asked another.
"I better not get charged a late fee on my mortgage," griped a third.