North America has the highest distribution of both unique Android and iPhone OS devices, according to AdMob’s latest metrics.

According to the report, 49 percent of unique iPhone OS devices in April of 2010 were located in North America. The continent is followed by Western Europe. The top five countries with the most unique iPhone OS devices in April 2010 were the United States at 44 percent, the United Kingdom at 9 percent. France, Canada and Japan all took percentages below 10 percent.

Still, the report found that Apple’s growth is taking place in the international markets. AdMob says that over the past 12 months, the iPhone platform has grown faster internationally than in North America. In Asia, the iPhone OS grew 474 percent on AdMob’s network. Australia and New Zealand saw 367 percent growth in the platform, and Western Europe 269 percent.

The data from a special section of the report measured unique devices, not ad requests. A unique device represents a device that has seen at least 1 ad request in the AdMob network in a given month.

According to AdMob, fully 75 percent of unique Android devices were located in North America in April 2010, followed by Asia at 12 percent and Western Europe at 11 percent. In the United States there was a 2 to 1 ratio of iPhone OS devices to Android OS devices. Worldwide the ratio of iPhone OS devices to Android devices was 3.5 to 1.

AdMob, one of the world’s largest mobile advertising networks, was recently acquired by Google for $750 million. The merger was approved last Friday, following an FTC investigation into possible antitrust conflicts.