Launched in 2007, Accertify provides a hosted software application that offers an extra level of security for transactions over any of the major payment networks, including American Express, Visa, MasterCard, Discover and PayPal.

The Illinois-based firm also offers its merchant customers the option to outsource their end-to-end fraud management process. Amex says the acquisition will enable it to broaden its fraud prevention services to merchants for online transactions on all networks, not just its own.

Bill Glenn, president, American Express Global Merchant Services, says: "While we already provide innovative fraud services for American Express transactions, merchants increasingly want single source platforms to address all types of transactions, regardless of the payment brand. We understand merchants' payment needs on a broader scale, and we're building on our commitment to expand e-commerce solutions and value-added services. Accertify will enable us to further increase trust and confidence in the online payment space."

Jeff Liesendahl, co-founder and chief executive officer of Accertify, will continue as president once the deal closes, which is expected to be in the fourth quarter, subject to customary conditions.

The value of the e-commerce market to card firms has seen Amex rivals MasterCard and Visa both make significant acquisitions in recent months. In April, Visa agreed a $2 billion deal to buy online payment and security outfit CyberSource and MasterCard followed up soon after with the £333 million purchase of DataCash.