A US financial industry group warned banks and other institutions to beware of cyber attacks Wednesday, after some firms reported sporadic problems with their websites.
The Financial Services Information Sharing and Analysis Center said it raised its cyber threat level from "elevated" to "high."
The group, which monitors cyber threats to the sector, cited "recent credible intelligence regarding the potential for DDoS and other cyber attacks against financial institutions."
DDoS stands for "distributed denial of service", attacks in which vast numbers of computers simultaneously attempt to contact the target networks, swamping their servers.
It also said a vulnerability reported in Microsoft's Internet Explorer browser was a factor.
"Members should maintain a heightened level of awareness, apply all appropriate updates... and ensure constant diligence in monitoring and quick response to any malicious events," the group said on its website.
On Tuesday, the Site Intelligence Group said a group of hackers calling themselves the "Cyber fighters of Izz ad-din Al Qassam" announced an attack on Bank of America and the New York Stock Exchange websites.
The group claimed they were in retaliation for the release of the controversial movie "Innocence of Muslims," which has led to massive protests across the Muslim world.
The NYSE declined to comment, but a source familiar with the exchange said the site was not affected.
Bank of America spokesman Mark Pipitone said the financial giant's website "is, and has been available throughout the day, although some customers may have experienced occasional slowness."
Asked about the reported attack, the spokesman said: "I can assure you we continuously take proactive measures to secure our systems."
JPMorgan Chase's consumer bank unit also reported some slowness.
"Some customers are having trouble getting on Chase.com. We're working on it and apologize for the frustration," a spokesman said.