San Francisco - Google is bringing its digital wallet to the iPhone in its latest attempt to upstage Apple on its own popular device.

Thursday's release of the Google Wallet app represents a challenge to the Passbook program that Apple has built into the iPhone's operating system.

Both Google Wallet and Passbook allow iPhone users to store loyalty cards from some merchants and scan coupons offering discounts.

Google Wallet also allows its users to send money and make purchases on some mobile websites by storing a debit or credit card account on the app. Payment information can also be stored in Passbook.

The arrival of Google Wallet on the iPhone comes a few days after a similar version was released to smartphones running on Google's Android software.

Google gives away Android for free, a factor that has helped siphon sales away from the iPhone by enabling other smartphone makers to sell their devices at cheaper prices. About three out of every four smartphones sold during the first half of this year ran on Android software, according to the research firm Gartner Inc. In comparison, the iPhone had a 16 percent share of the worldwide market.

The higher-priced iPhones tend to attract more affluent consumers who are more likely to spend money through their devices, one of the reasons that Google is eager to connect with Apple customers.

Google has previously sought to outshine Apple's built-in iPhone apps with its digital maps and a mobile version of its Chrome browser.

By getting people to use its services on as many devices as possible, Google hopes to make more money by selling more digital advertising.