MOUNTAIN VIEW -- Look out, Siri. Google Now is now available on the iPhone and iPad.

Google announced Monday that its version of an online personal assistant, originally introduced last summer for smartphones running the latest version of Android, can now be used on Apple phones and tablets where Apple's own Siri has charmed and occasionally frustrated millions of users.

Siri and Google Now work differently, but each is designed to serve as a handy starting point for answering questions and providing useful information to people who use mobile computing gadgets to manage their daily lives.

While analysts say the voice-activated Siri could someday be a competitive threat to Google's vaunted Internet search engine, Google's decision to offer Google Now through the Apple iTunes store -- as an update to its Search app -- is viewed as a challenge to Siri on its home turf.

Technically, Siri isn't a search engine, but it can search for information by using Google and other online resources. It can also send messages and perform other services in response to voice commands, although its occasional mistakes can be unnerving for users.

Google Now relies more on text and pictures than spoken words, as it attempts to deliver personalized information that's helpful even before a user requests it. It proactively serves up brief reports on such topics as local weather, sports scores, flight status updates or driving directions to upcoming meetings -- based on what Google learns about users from their GPS location, recent Internet searches, online calendar items and emailed confirmations from airlines.

The two companies have been locked in a fierce rivalry for the attentions of mobile-device users. Google's Android software is currently the most widely used smartphone operating system in the world, and some of the most popular apps on the iPhone are Google's Search, Maps and YouTube.

Apple has had mixed results in developing its own apps to compete. An Apple Maps app was plagued with problems, but analysts say Siri could replace Google's profitable, advertising-supported search engine for some consumers.

When asked for an opinion about Google Now, Siri told a reporter for The Associated Press: "If it's all the same to you, I'd rather Google later."

Contact Brandon Bailey at 408-920-5022; follow him at Twitter.com/BrandonBailey.