The pair first teamed up in 2010 to offer cardholders reward redemptions for virtual goods and games produced by Zynga, the developer of the popular Farmville Facebook game.

For the new offering, Farmville players will be encouraged to 'plant' an interactive Serve 'money tree' in their farms. They will then be prompted to apply for a Zynga Serve pre-paid card which can be topped up through cash or debit or credit cards. Purchases made in-store or online with the card see users collecting in-game rewards that appear in real-time.

At launch, only the first five purchases of $25 or more on the Zynga Serve card will earn Farm cash but as the programme expands all buys will receive rewards tracked through an in-game dashboard.

Dan Schulman, president, enterprise growth, American Express, says: "As the commerce landscape continues to change, and online and offline spending converges, Serve is focused on partnering with companies like Zynga to create unique value for our customers in the environments they love."