Telling users the company is performing maintenance on its security measures, the pop-up asks for Social Security numbers, mother's maiden name and date of birth.
"Please note that this fraudulent activity may be the result of a computer virus and is not a part of the American Express website. If you received this pop-up box, your computer may have this virus. The use of both anti-virus software and a firewall to protect your PC are strongly recommended," the credit card company warned on its website. "If you received this pop-up box and entered your information, please contact American Express by calling the number on the back of your card."
The site also gives a crash course on phishing and how to avoid becoming a victim. The advisory also warns users not to open attachments to unfamiliar emails since they may contain key stroke loggers or other malware.
The company also asked users who receive phishing emails to send them to Anti.Phishing.Team@aexp.com.
American Express does not ask customers to send it user identification numbers, passwords or social security numbers in emails, according to the company website.
Reports of this latest phishing scam came a few days after MasterCard and Visa were notified of a security breach at a U.K.-based retailer.
According to published reports, the breach occurred at a U.K. online retailer, leading to thousands of holders of both companies' cards to cancel their accounts. As many as 4,000 U.K. customers were affected, according to reports.