Apple Inc sold more than 600,000 of its newest iPhone after a single day of preorders, blowing away most expectations and sending its shares up more than 2 percent on Wednesday.

Analysts said that put the new iPhone on track to surpass sales of the previous model, about 1 million units of which moved in its first three days.

Apple, which also makes the iPad and iPod, said it and carrier partner AT&T Inc had been forced to turn away many potential customers after the surprising volume of online orders triggered malfunctions in its order and approval systems.

Apple apologized for the snafu, asking iPhone faithful to try again. But AT&T said the availability of inventory would determine if it can resume taking orders.

Apple unveiled the slimmer, $199 iPhone 4 last week, kicking off its fastest-ever global product roll-out to try to stay a step ahead of rivals like Google Inc in a red-hot smartphone market. (For a Reuters Insider story see http://link.reuters.com/fas22m)

The device boasts a higher-quality screen and better battery life, video chat via Wi-Fi, and a gyroscope sensor for improved gaming.

We apologize to everyone who encountered difficulties, and hope that they will try again ... once the iPhone 4 is in stock, Apple said in a statement.

AT&T, the exclusive U.S. provider for iPhone, said orders of the iPhone 4 on Tuesday -- the first day of online preorders -- were 10 times higher than for the iPhone 3GS last year.

It said it chalked up more than 13 million visits to its website on Tuesday, including customers checking to see if they were eligible to upgrade to a new phone.

The iPhone 4 is scheduled to hit stores on June 24. Apple's website said on Wednesday morning that preordered phones would be shipped by July 2. It had previously promised shipments by June 24.

Apple shares were up 2.4 percent at $265.92 on the Nasdaq in early afternoon.

It's a two-year replacement cycle. There are going to be a lot of people upgrading from the 3G, as well as incremental additions from new customers, Broadpoint Amtech analyst Brian Marshall said.

(Reporting by Gabriel Madway and Sinead Carew, writing by Edwin Chan, editing by Matthew Lewis)

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