Apple Chief Executive Officer Tim Cook extended his buying spree with the iPhone maker acquiring speech-recognition software company Novauris Technologies to help improve its Siri service.

Apple said Thursday it acquired the U.K.-based software company without disclosing how much it paid. The team behind Novauris have worked for companies including Dragon Systems, which makes dictation software.

The deal fits with Apple's approach of acquiring smaller companies then swallowing the teams and technology before releasing related features in future products. The strategy contrasts with peers such as Google and Facebook, which have been spending billions of dollars on acquisitions.

Kristin Huguet, a spokeswoman for Apple, confirmed the acquisition that was initially reported by TechCrunch.

"Apple buys smaller technology companies from time to time, and we generally do not discuss our purpose or plans," Huguet said.

Apple made 15 acquisitions in its last fiscal year, compared with five known deals the previous year. In its last quarter, Apple spent $525 million buying other companies, almost double what it spent a year earlier.

In addition to Novauris, Apple has acquired PrimeSense, the maker of motion-tracking chip technology; Topsy Labs, a data-analytics firm; and mapping software company Broadmap.

Siri is the digital assistant Apple introduced in 2011 so customers can ask for driving directions or check the weather by speaking a command in to their phones. The service was born out of Apple's 2010 acquisition of a company of the same name.