SAN FRANCISCO -- Apple updated its online store on Tuesday to begin offering unlocked models of iPhone 4 in the United States for the first time.

"The unlocked iPhone 4 requires an active micro-SIM card that you obtain from a supported GSM wireless carrier," said Apple in the product description. The iPhone requires a smaller version of the standard SIM (Subscriber Identity Module) card.

Started at $649 for the 16 GB version, the unlocked model is a GSM phone, which means, in the United States, the phone runs on networks of T-Mobile or AT&T, although it can only send data over T-Mobile's old EDGE network, not its 3G network and the faster HSPA+ network.

Verizon and Sprint, the other two major US wireless carriers, both use CDMA networks that do not use SIM cards.

The iPhone 4 has been sold unlocked in other countries. For frequent international travelers, an unlocked iPhone means they just need to pop in a micro-SIM card for whichever country they are going, avoiding provider's high international fees.

According to Apple, iPhone sales grew 113 percent year over year in the second quarter of its fiscal 2011, reaching a record high of 18.65 million units.