The retail banking website of J.P. Morgan Chase & Co. (JPM) on Tuesday has come under a so-called “denial of service” attack, rendering it unusable for customers, a bank spokesman confirmed.

The site first slowed earlier Tuesday, and in the afternoon it became unavailable. The bank is responding with increased security measures for the website, chase.com. The spokesman said no customer data had been compromised, but didn’t say when the site would be fully restored.

The bank’s mobile-banking applications are working, and branches and automated teller machines aren’t impacted.

The bank, the nation’s largest by assets, told customers in a Twitter message Tuesday afternoon that it is experiencing “intermittent issues,” followed by another message stating that the bank is working “on getting Chase Online back to full speed.”

On the website, the bank posted: “Our website is temporarily unavailable. We’re working to quickly restore access. Please log on later.”

Banks have been increasingly hit by cyberattacks over the last two years, including DOS attacks that increase the volume of website hits, slowing access to the sites by customers. Banks have been preparing in recent days for a new wave of DOS attacks, according to a banking industry source, including strengthening their firewalls.

Citigroup Inc. (C) said in its annual earnings filing with the Securities and Exchange Commission last month that it, like other banks, was the victim of several cyberattacks in 2012 and previous years, and that it managed to detect and respond to these incidents “before they became significant.” The attacks nevertheless “resulted in certain limited losses in some instances.”

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