Yahoo has acquired Actionality, a German mobile advertising technology company, for an undisclosed sum.

Actionality provides software technology that inserts ads in content for mobile devices, such as mobile phone games. The company claims that it has simplified in-game branding and advertising to the point that it's a one-click process.

A Yahoo spokesperson said in an e-mail that no one was available to discuss the acquisition but confirmed the deal "as part of Yahoo's ongoing focus on becoming No. 1 in mobile advertising."

In January, Yahoo launched oneSearch, search technology tailored to mobile devices, and Go for Mobile 2.0, a way to access Yahoo content on Windows Mobile devices.

In February, Yahoo launched a mobile device display advertising system on its Mobile Web service in 19 countries.

In March, Yahoo introduced Yahoo Mobile Publisher Services, a suite of services to help advertisers, publishers, and network operators generate revenue from mobile content and advertising. The services include Yahoo Mobile Ad Network, Mobile Content Engine, Mobile Media Directory, and Mobile Site Submit.

"Yahoo is being very ambitious and will face strong competition in the mobile advertising market, which is still very much up for grabs," said Eden Zoller, an analyst with market research firm Ovum, in an April research note. "Google tends to be viewed as the lead contender in terms of online brands moving into mobile advertising, mainly because of the huge clout carried by its brand among both consumers and the advertising community. However, we think it unwise to underestimate Yahoo. The mobile publisher advertising network should be viewed alongside Yahoo's other activities in this space, which are ramping up quickly."

The Actionality deal marks Yahoo's fourth acquisition in the last eight months. Earlier this year, Yahoo acquired MyBlogLog, Right Media, and Rivals.com.

Google has made 15 acquisitions this year, two of which -- DoubleClick and Postini -- are still pending.

AOL and Microsoft have also made notable acquisitions in the mobile arena: In May, AOL acquired Third Screen Media, a mobile advertising technology firm. That same month, Microsoft acquired French mobile advertising company Screen Tonic.