SAN JOSE, Calif. - Apple Inc. has issued a software update that the company said will create problems for iPhones that have been modified to use a cellular carrier other than AT&T Inc.

Apple warned earlier this week that the iPhone update -- which adds access to the iTunes Wi-Fi Music Store and fixes some security flaws -- could wreak havoc or permanently disable phones that carried unauthorized programs designed to unlock the handset from the exclusive use of AT&T's network.

Several gadget enthusiast websites, including Gizmodo and Engadget, as well as online postings from hacker communities reported that, depending on which unlocking program was used, certain modified phones no longer worked after they installed the software update. In some cases, the phones would work, but only with the original SIM card that ties the phone to AT&T.

Some sites also reported that iPhones equipped with other unofficial, third-party applications became disabled after the update.

Some hackers had characterized Apple's warning as "a scare tactic.'' Despite Apple's history of playing cat-and-mouse games with hackers concerning other products, Apple officials insisted they were "not proactively'' trying to make hacked iPhones useless.

It was unclear how many iPhone owners had unlocked their phones, but the programs -- including several that can be downloaded free -- appeared to be particularly popular with European consumers. Apple isn't selling the iPhone in Europe until November, so the unlocking software allowed Europeans who bought iPhones in the United States to use the phones.

Installing Apple's latest iPhone update is optional.