Advertisers take notice. Views of online advertisements have been gradually migrating on mobile devices as shown by a new report issued on Thursday by Internet advertising firm Millennial Media.

According the New York-listed company, tablets, foremost of which Apple's iPad and Samsung's Galaxy Tab, have been determined as the main gadgets used by online visitors in viewing Web-based ads.

Ad impressions on the emerging online tool accounted for 20 per cent of the total advertisements that online surfers have either accessed or viewed in the first three months of 2012, a shoot up of 5 per cent from the same period last year.

"Tablets and other non-phone connected devices have established a clear role in the growing mobile ecosystem," Jamie Fellows, Millennial Media senior vice president for products, said in a statement that accompanies the report titled Q1 Mobile Mix.

"The substantial quarterly growth of impressions in this category is tied to both increased usage and adoption, and we expect this trend to continue in the foreseeable future," Ms Fellows added.

The findings confirmed earlier assertions by Internet giant Google that a shift in online advertising has already transpired and companies wishing to increase their Web visibility would do well to reconfigure their sites on mobile devices such as smartphones and tablet computers.

Also, the Millennial Media study came out as Apple and other firms manufacturing mobile devices on the Android platform have been raking in huge revenues on millions of gadgets sold in the past few years.

As of the end of March this year, iPad topped the list as the bestselling and highly preferred tablet computer in most markets around the world and was closely followed by the Galaxy Tab, which Samsung sells in various screen sizes, the study said.

Amazon's Kindle Fire, which retails the cheapest at only $US199 per unit, came in third to beat out other Android-based tablet computers.

Google's free Android mobile operating system was tagged by the Q1 Mobile Mix report as the most widely-used platform in online ad viewing, which in the first quarter gobbled up 49 per cent of total shares in the period.

Apple, however, still rose as the number one mobile device manufacturer and the top smartphone in the period, cementing the company's reputation as one of the tech world's most visible and valuable brands.

Right behind the company was Samsung, which the Millennia report said counts four different mobile phone variants in the study's list of Top 20 Mobile Phones.

Unlike Apple, which sells only the iPhone, Samsung offers a mixed line of entry-level and high-end Android phones led by the Samsung Nexus, the Galaxy Note and the Galaxy SII, which welcomed its replacement earlier this month, the Galaxy SIII.

The new study also highlighted users' growing preference on video ads, which registered considerable impression spikes in the last 12 months leading to the end of March this year.

Gaming applications as usual took the top spot as best vehicle there is for online advertisers to promote their products as impressions generated by the apps increased by 10 per cent when compared to its performance in the previous quarter.