Android increased its lead over Apple's iOS in April as the top mobile platform tracked by ad network Millennial Media.

Releasing its April MobileMix report today, Millennial Media found that Android accounted for 53 percent of all ad impressions on its network last month, a gain of 10 percent from March. That marked the fifth consecutive month in which Google's mobile OS was the leader.

Android's growth in April came at the expense of both iOS and Research In Motion's BlackBerry OS.

For the month, the share of ad impressions for iOS fell to 28 percent from 31 percent in March. Apple had seen a surge in March following the debut of the Verizon iPhone. RIM's share inched down to 16 percent from 18 percent in March.

Looking beyond the OS to the mobile-device makers themselves, Apple retained its dominance last month, accounting for 32 percent of all ad impressions--a number that included the iPhone, iPad, and iPod Touch.

Second among mobile devices for the third month in a row was Samsung with a 15 percent share. For the first time, Samsung's Nexus S smartphone found its way on the top-20 list of mobile phones, landing on the No. 4 spot.

HTC took third place with 11 percent of all impressions, an increase of 10 percent from the previous month. With seven of its devices among the top-20 mobile phones, the company scored the highest number of handsets on the list.

RIM followed in fourth place with 10 percent of all ad impressions for the month.

Smartphones continued to surge as a result of rising consumer demand. For the first time, the top-20 mobile devices tracked by Millennial Media included only smartphones and no feature phones. Since April 2010, ad impressions among all smartphones have risen by 51 percent.

Overall, smartphones accounted for 68 percent of all the mobile devices tracked last month, with feature phones snagging 16 percent share and connected devices (iPad, iPod Touch, mobile game consoles) grabbing 16 percent as well.