JPMorgan Chase & Co., the biggest U.S. bank, said it increased defenses against computer hackers after an attack against the industry this month.

The lender is working with U.S. authorities to determine the scope of the assault and is taking additional steps to safeguard confidential information, Patricia Wexler, a spokeswoman for the New York-based bank, said today in an e- mail, declining to give specific examples. JPMorgan will contact any customers that might have been affected, Wexler said, adding that the firm hasn’t seen unusual fraud levels.

JPMorgan was among at least five banks targeted in the coordinated attack on financial institutions in recent weeks, a U.S. official said yesterday. The assault led to the theft of customer data that could be used to drain accounts, according to another person briefed by U.S. law enforcement. Both asked not to be identified because U.S. probes are continuing.

Chief Executive Officer Jamie Dimon, 58, has warned shareholders in annual letters that hackers’ efforts to breach the bank’s computers were growing more frequent, sophisticated and dangerous. The firm expected to boost yearly spending on cybersecurity to about $250 million by the end of 2014, with 1,000 workers dedicated to the effort, he wrote in April.

Never-Ending Battle

“We’re making good progress on these and other efforts, but cyberattacks are growing every day in strength and velocity across the globe,” Dimon said in that letter. “It is going to be a continual and likely never-ending battle to stay ahead of it — and, unfortunately, not every battle will be won.”

During attacks this month, hackers targeted banks’ customer and employee information, said a third person involved in the investigation, who was also briefed by the government. The theft involved gigabytes of data, said several people familiar with the case. The scale indicates a potential for significant financial fraud.

JPMorgan customers won’t be held liable for fraud related to the attacks, and should reach out to the bank if they see suspicious activity, Wexler said.

Past thefts of financial information have mainly involved retailers or consumers’ personal computers. Stealing data from big banks is less common because they have elaborate firewalls and security systems.