The hackers, believed to be activists in the Middle East, were highly knowledgeable about the defensive equipment used by the banks and likely spent months on reconnaissance, said several researchers interviewed by Reuters, who viewed the assaults as among the strongest and most complex the world has seen to date.

In the past two weeks, customers of top U.S. banks including Bank of America Corp, JPMorgan Chase & Co, Wells Fargo & Co, U.S. Bancorp and PNC Financial Services have reported having trouble accessing their websites, as unusually high traffic volumes appeared to crash or slow down the systems.

No thefts have been tied to hacked sites, but an untold number of customers were not able to pay bills or transfer money from their computers, leaving banks with remediation expenses and customer irritation as the biggest costs.

Researchers said the hackers used groups of compromised computers, known as botnets, which are inexpensive to rent for short periods. What made these botnets much more powerful was that they were made up of web servers that had been taken over, instead of mere personal computers.

"Tens of thousands" of servers are involved, said Tom Kellermann, vice president of major security vendor Trend Micro.

The FBI declined to comment on its investigation of the attacks. The banks either declined to comment or noted that most customers have been able to log into their accounts.

"It's fairly large, but it's something financial institutions are accustomed to dealing with," said Doug Johnson, vice president of the American Bankers Association trade group.

Sources familiar with the bank attacks have previously told Reuters that they could be part of a year-long cyber campaign waged by Iranian hackers against major U.S. financial institutions and other corporate entities.

Senator Joseph Lieberman, chairman of the Senate's Homeland Security and Governmental Affairs Committee, has also blamed Iran's much-improved cyber forces on the bank website outages.

A group that calls itself the Cyber Fighters of Izz ad-din Al Qassam has claimed credit for the recent bank attacks, declaring them a protest against the anti-Islam video posted on YouTube and stoked violent protests across the Muslim world.

The latest attacks against the banks have thrown as many as 30 million electronic packets per second at the websites, at times overwhelming both the banks and the additional technical resources being moved into place to counter the attacks.

That much volume "would overwhelm almost anyone, including large telecommunications companies," said Scott Hammack, chief executive of Prolexic Technologies, which specializes in warding off "denial of service" attacks. Prolexic's clients include several of the biggest banks, though Hammack declined to name which ones.

Experts said that high-volume denial-of-service attacks were becoming more common even before the latest bank assaults and would continue to increase in sophistication as well.

"This entire episode speaks to the need for banks, or any business operating online, to be prepared for this type of availability attack," said Dan Holden, director of research at security firm Arbor Networks.

(Reporting by Joseph Menn in San Francisco. Additional reporting by Jim Finkle in Boston and Rick Rothacker in Charlotte, N.C.; Editing by Bernard Orr)

• A Permanent Economic Emergency