Microsoft’s Windows Phone mobile operating system is making slow progress in the US, but Strategy Analytics forecasts that the software will account for 4% of the smartphone market there this year. The figure equates to 6m Windows Phone-based smartphones sold this year and the research firm describes the growth as a “steady increase” from its 3% share in 2011. However, growth is slow going for the firm and illustrates the task facing Microsoft as it attempts to wrestle market share from Apple’s iPhone and Google’s Android OS. Overall, the research firm estimates that 123m smartphones will be sold in the US this year, a 21% increase year on year from the 102m sold in 2011.

“Microsoft is making a determined push to crack the US because it is the most valuable and influential smartphone market in the world. To grow further, future versions of Windows Phone 8 will need to dramatically improve support for advanced technologies like multi-core chipsets, enhance the Marketplace app store, expand the number of phone models available from major partners like Nokia or Samsung, and consider reducing the license fees it charges per unit to smartphone makers," says Strategy Analytics executive director, Neil Mawston.

The research firm asserts that Samsung, HTC and Nokia will drive Windows Phone-based device sales this year, despite the latter two vendors struggling to gain traction in the country. HTC has suffered a well-documented fall from grace since briefly reigning as the top US smartphone manufacturer, and CEO Peter Chou recently conceded that it will be difficult to recover its share in the country. Meanwhile, Nokia recently slashed the price of its high-end Lumia 900 smartphone amid claims that it has only shifted 330,000 units of its Lumia line since its debut four months ago. Contrastingly, Samsung is enjoying a purple patch of late, and analysts estimate that the South Korean firm shifted as many as 50m smartphones globally during its latest quarter, though Strategy Analytics does not indicate which vendor will account for the most Windows Phone sales.

Strategy Analytics’ forecast is more optimistic that Nielsen’s, with the research firm claiming that Windows Phone accounted for just 1.3% of US smartphone sales in Q2. Separate research from Piper Jaffray analyst Gene Munster further illustrates the challenge facing Microsoft in the highly competitive US market. The analyst claims that almost two-thirds of US phone owners expect their next device to be an iPhone, with the widely-anticipated new iteration almost certainly set to boost Apple’s market share further this year.