Two in every three smartphones sold by the top three US mobile operators in Q1 related to iPhone activations, according to new analysis by Wireless Intelligence.

The research house calculated that there were 9 million iPhones sold in the US in the quarter via Verizon Wireless, AT&T and Sprint, accounting for around two-thirds of the estimated 13.5 million total smartphones sold by the three operators in the period.

Although Android devices are now thought to account for over half of all US smartphones in service, the figures suggest that customers signing-up to the top three operators are increasingly opting for the Apple device over alternatives.

“It's clear that the iPhone has wedged itself a near-majority share of gross smartphone additions,” commented Wireless Intelligence analyst Will Croft. “The overall growth in iPhone sales has significantly and consistently eroded Android's share of sales [in the US].”

AT&T was responsible for almost half of US iPhone sales (48 percent / 4.3 million) in Q1, which accounted for 78 percent of the US number-two operator’s 5.5 million smartphone sales. Verizon Wireless and Sprint activated 3.2 million and 1.5 million iPhones each, accounting for 51 percent and 76 percent of smartphone sales, respectively, according to Wireless Intelligence calculations.

AT&T offered the iPhone on an exclusive basis for two-and-a-half years until February 2011 when Verizon Wireless launched a version compatible with its CDMA network. Sprint began offering the device in October 2011 (coinciding with the launch of the iPhone 4S), making Q1 2012 its first full quarter of iPhone sales.

However, the total 9 million US iPhone sales in Q1 was below the 13.6 million iPhones sold in the country the previous quarter (Q4 2011), which included the launch of the iPhone 4S and key holiday shopping season – and contributed to a record 37 million iPhone activations worldwide. In that quarter, the US accounted for 38 percent of total iPhone sales, but the country made up just 26 percent of sales in Q1 2012.

This was a result of Apple ramping up iPhone sales efforts in non-US markets during the period, notably in Asia-Pacific where Apple reported a 114 percent year-on-year increase in regional revenue to US$10.2 billion, making it the firm’s second-largest regional market after the Americas (US$13.2 billion).

“The US is a victory for Apple, but the importance – and growth – of operator share outside of the US remains key, particularly in Asia,” says Croft. “The non-US share of iPhone shipments, currently 74 percent, has risen sharply from 55 percent in 2008 and 67 percent in 2010. For Apple, this is the critical opportunity, particularly in its two fastest-growing markets, China and Japan.”