RHONDA SCHWARTZ, PIERRE THOMAS and LEE FERRAN report:

The Department of Homeland Security and the FBI are cautioning American government and financial institutions that they could be targets of a wave of cyber attacks Tuesday from Anonymous-linked hacktivists in the Middle East and North Africa.

"The attacks will likely result in limited disruptions and mostly consistent of nuisance-level attacks against publicly accessible web pages and possibly data exploitation," says an unclassified memo from the Department of Homeland Security, first obtained by the cyber security blog KrebsOnSecurity.com.

In another memo, this one from the FBI's Cyber Division and obtained by ABC News, 140 banks are listed as potential targets for the potential cyber attack campaign known as "OpUSA." Threats against the targets were originally made weeks ago and posted publicly online in a rambling missive that also denounced American "war crimes" in Iraq, Afghanistan and Pakistan.

Both U.S. government and industry analyses of the OpUSA threats have connected them to OpIsrael, a widespread but reportedly largely ineffective cyber attack targeting Israeli government and private websites last November. As in that attack, OpUSA hackers are expected to use distributed denial of service (DDoS) attacks to flood target websites with illegitimate traffic, potentially knocking them offline, a Department of Homeland Security official said.

One industry analysis says that due to the "hive mindset" of groups like Anonymous, the attack's effectiveness could be contingent on its popularity and perceived success.

"Similarly, if the central actors appear to be largely failing in their efforts, other Anonymous actors may decide not to join in this operation," the analysis says.

Cyber security expert Mikko Hypponen of F-Secure told ABC News he expects that "something's going to happen," but likely not more than some websites being defaced or briefly knocked offline by the DDoS attacks.

If it proves correct, Hypponen's prediction would be a far cry from OpUSA's original promise to wipe the U.S. "off the cyber map."

ABC News' Brian Ross contributed to this report.