The latest problems on Chase.com came as intelligence officials said for the first time on Tuesday that cyber attacks and cyber espionage have surpassed terrorism as the top security threat facing the United States.
JPMorgan and other major U.S. banks, including Bank of America Corp and Citigroup Inc, have recently warned their investors that their sites have been attacked and that the assaults could continue.
JPMorgan spokesman Michael Fusco said the company continued to work late Tuesday to restore normal service. He declined to say how long the Chase.com site had been down during the day.
Attempts by Reuters journalists to access the site showed mobile apps worked but efforts to log in through a personal computer brought up a notice that the site was down.
The assessment of cyber attacks came in an annual "worldwide threat" briefing and was reinforced by testimony from James Clapper, the Director of National Intelligence, to the U.S. Senate Intelligence Committee.
Cyber attacks on companies, particularly on U.S. banks, are getting worse, Army General Keith Alexander, head of the U.S. military's Cyber Command, said in a separate hearing of the Senate Armed Services Committee.
Starting in September, a hacker activist group called the Izz ad-Din al-Qassam Cyber Fighters said it was targeting major banks with denial-of-service cyber attacks. The attacks can disrupt service by deluging Web sites with high traffic.
In December, customers of Wells Fargo & Co had trouble using the bank's site for at least four days.
(Reporting by David Henry; Editing by Edwina Gibbs)