WASHINGTON — Organized cyber-gangs in Eastern Europe are increasingly preying on small and mid-size companies in the United States, setting off a multimillion-dollar online crime wave that has begun to worry the nation's largest financial institutions.

WASHINGTON — Organized cyber-gangs in Eastern Europe are increasingly preying on small and mid-size companies in the United States, setting off a multimillion-dollar online crime wave that has begun to worry the nation's largest financial institutions.

A task force representing the financial industry sent out an alert Friday outlining the problem and urging its members to implement many of the precautions now used to detect consumer bank and credit card fraud.

"In the past six months, financial institutions, security companies, the media and law enforcement agencies are all reporting a significant increase in funds transfer fraud involving the exploitation of valid banking credentials belonging to small and medium-sized businesses," the confidential alert says. The alert was sent to members of the Financial Services Information Sharing and Analysis Center, an industry group created to share data about critical threats to the financial sector.

Because the targets tend to be smaller, the attacks have attracted little of the notoriety that has followed larger-scale breaches at big retailers and government agencies. But the industry group said some companies have suffered hundreds of thousands of dollars or more in losses.

In many cases, the advisory warned, the scammers infiltrate companies in a similar fashion: They send a targeted e-mail to the company's controller or treasurer, a message that contains either a virus-laden attachment or a link that — when opened — surreptitiously installs malicious software designed to steal passwords. Armed with those credentials, the crooks then initiate a series of wire transfers, usually in increments of less than $10,000 to avoid banks' anti-money-laundering reporting requirements.