Visa Inc. and MasterCard Inc. severed ties with Backpage.com after law enforcement officials raised concerns that the classified advertising website “promotes prostitution and facilitates online sex trafficking.”

According to a letter sent Monday to Visa and MasterCard by Cook County, Ill., Sheriff Thomas Dart, in just one month Backpage.com posted more than a million ads in its “adult escort” section in the United States, with most of the ads containing prices, hotel locations, nude photos and, in some instances, video.

Visa said it stopped allowing its cards to be used on the site Wednesday, following a similar move by MasterCard on Tuesday and by American Express Co. earlier this year.

Backpage.com is the second-largest ad listing service on the Web, next to Craigslist.

The site for years has drawn the scrutiny of government and law enforcement officials who say the website makes it easy for pimps to exploit sex workers and fuels the illegal sex trafficking industry.

In a statement, Visa said it stopped processing payments for Backpage.com because it prohibits its network to be used for illegal activity.

Backpage.com did not respond to requests for comment.