Microsoft's Ad Exchange hands advertisers and publishers the ability to buy and sell digital inventory through an automated RTB auction on a cost-per-impression basis.
Dennis Buchheim, general manager of scale display at Microsoft, said the roll-out will be extended further to include Canada, France and Germany in the coming months.
In addition, the company announced that all Windows Phone 7 in-app ad inventory is now available globally via the Microsoft Advertising Exchange for Mobile.
Buchheim said: "We've seen very encouraging results so far in the US. Advertisers are seeing improved ROI on their campaigns, while publishers are achieving higher yields on their inventory – all in an open, premium marketplace.
Microsoft has also unveiled an interactive ad platform for the Kinect Xbox 360, which it claims will transform TV advertising into a service controlled by voice and gesture commands.