Monster.com, which is owned by New York-based TMP Worldwide, sent an e-mail to people who have signed up to find jobs on their site, warning them of the potential for false job postings and identity theft.
"This is very rare," said Monster.com president Steve Pogorzelski. "This e-mail was not in response to any recent identity theft problem."
However, he acknowledged there had been attempts to victimise job seekers in the past.
Monster.com warned job seekers to be wary of postings that come from companies outside the US because of a disproportionate number of fake listings from Eastern Europe, Pogorzelski said.
People are posing as fake employers, using real company names or making them up, and asking applicants for sensitive information - such as social security, credit card and bank account numbers - under the guise of doing a routine background check.
Pam Dixon, a research fellow at the Denver-based Privacy Foundation, a research and advocacy group for consumer privacy, said the problem was getting worse as more and more people lose their jobs and flock to the internet to look for new ones.
Dixon said she had heard from victims "from every single job site" on the internet, at least 50 in the past few years. She has contacted the US Federal Trade Commission about the matter.
"Con artists pose as legitimate employers, like Ford or IBM, and they'll do e-mail or phone interviews," said Dixon, and ask for the information for pre-hire background checks.
They are also grabbing CVs posted on Web sites and selling them to spammers or others who can then use the information on them for identity theft, she said.
Identity thieves are contacting job seekers and asking for sensitive information after seeing their CVs online.
Monster.com warns job seekers not to give out their social security number, credit card or bank account numbers or other personal information, even for claims of background checks.
Legitimate employers eventually will need that information for security checks, particularly under national security requirements after the attacks of 11 September, Dixon said.
Dixon and Beth Givens, director of the US non-profit consumer advocacy group Privacy Rights Clearinghouse, recommended that job seekers first verify that the purported employer is really who they say they are and wait until after an in-person interview before releasing additional information.
"We're in a very difficult job market, so it's easy to twist the arms of applicants" to give up personal information, said Givens. "They're more easily duped in the economy we have now than if employers were fighting over applicants."
Monster.com, which has more than 24 million CVs and hundreds of thousands of jobs posted on its website, screens postings for suspicious details that might indicate they are false, Pogorzelski said.
Representatives of HotJobs.com, which is owned by Yahoo! and Chicago-based CareerBuilder, two other top US job sites, could not be immediately reached for comment last Friday.