In a move designed to compete more closely with Google' Android platform, Apple, is buying mobile application developer Siri for its technology that lets users perform voice-activated Web searches from their smartphones.

Siri, a start-up based in San Jose, makes a voice recognition and search application it describes as a virtual personal assistant, and began offering it for the iPhone this year. Users speak commands, and the application translates and uses search algorithms to find the answers. Siri partnered with companies including Citysearch, OpenTable, and Taxi Magic to get results.

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Siri had raised a total of $24 million from investors prior to its sale to Apple, including Menlo Ventures and Li Ka-Shing, a Chinese billionaire who has also invested in Facebook. The acquisition agreement has been signed but the deal hasn't closed yet, according to Shawn Carolan, a managing director at Menlo Ventures.

Carolan, who sits on Siri's board, said the Apple offer "had to be very compelling" for the executive team and the board to accept. He declined to disclose the terms.

Industry observers believe Apple will at some point want to offer an alternative to Google's search service on the iPhone, the iPod Touch and the iPad. Google has invested heavily in voice command search, location-based search and advertising and in visual recognition search.

Siri's management team includes artificial intelligence experts, and the product grew out of the SRI Cognitive Assistant that Learns and Organizes (CALO) Project, the largest artificial intelligence project in the U.S., funded with $150 million from DARPA, the U.S. Defense Advanced Research Projects Agency.

For Siri, the Apple acquisition means the start-up will be able to scale its application. Right now, Siri is built for the iPhone 3GS and the iPod touch and works only in the U.S. With Apple's support, Siri will be able to push its application to other devices and expand outside the U.S.

Apple CEO Steve Jobs began describing his company as a mobile devices maker this year, and in recent months has increased the number of acquisitions in order to add technologies and features to its gadgets. The Cupertino-based Apple competes with Research In Motion's BlackBerry and devices running Google's software in smartphones, a segment that's outpacing the rest of the handset market.

Apple spokesman Steve Dowling said that Apple "buys smaller technology companies from time to time" and generally doesn't comment on their plans.

On Tuesday, Apple announced the acquisition of chip designer Intrinsity, after buying mobile-ad network Quattro Wireless in January.