Expedia will set aside $4 million to $6 million to compensate for fraudulent purchases on its Web site made by what appears to be an organized and professional credit card fraud ring, the company said. The site has seen an increase in the number of purchases made with fraudulent or stolen credit card numbers over the past few months, spokesman Erik Blachford said. Blachford said the purchases were organized and made by professional criminals, but would not elaborate. The company is working with law enforcement agencies to investigate the purchases. The stolen credit cards were not obtained from Expedia's customers, and the Web site remains secure, Blachford said. The company has since added more anti-fraud measures to help it verify credit card purchases. As part of Expedia's agreement with airlines, Expedia must pay face value for any fraudulent ticket purchases. Expedia.com, which was spun off from Microsoft Corp. last year, fell $1 to close at $19 on Nasdaq.