AT&T is to begin nationwide sales of its first LTE smartphones from this Sunday, as the company also switches on its network in four more markets.

It is set to offer HTC’s Vivid device (pictured), as well as a version of Samsung’s Galaxy S II which supports LTE connectivity. Both are powered by the Android platform.

The company has extended LTE coverage to Boston, Washington DC, Baltimore and Athens (Georgia). This takes its total to nine cities, with the intention to reach 15 markets with a combined 70 million population coverage by the end of 2011.

AT&T said that the launch of the smartphones means it has exceeded its targets for both “4G” devices and Android handsets. It had intended to offer 20 4G terminals – although it also includes HSPA+ devices in this group – and has now launched 22, while its Android target was 12 – the total is now 21.

The HTC device is priced at US$199.99 with a two-year contract, while Samsung’s Galaxy S II Skyrocket is $249.99, again on a two-year deal.

In a statement, AT&T said that it is “the only US carrier providing 4G service using both HSPA+ and LTE,” and that “customers get a faster and more consistent 4G experience, even when outside of an AT&T LTE area.”

It also argued that “customers of other carriers that have transitioned to 4G LTE without further speed upgrades to their existing networks are likely to see a jarring drop-off in speeds when they move out of LTE coverage.”

AT&T launched its first LTE services in September 2011, with initial availability in five markets. It trails rival Verizon Wireless, which started its rollout of LTE earlier, with its 4G network covering 165 cities.