Production problems could cause Apple to delay the release date for its iPhone 5S "two to four weeks" according to Citigroup analyst Glen Yeung.

The iPhone 5S still hasn't been officially announced but it was commonly believed a new device would come around a year to the day that the iPhone 5 was released. That no longer seems to be the case however.

The iPhone 5S was rumored for an early September release date to go up against new Android phones issued by Samsung and Sony.

Yeung said the delay is due to "display issues," but would not elaborate according to KnowYourMobile.com. It's believed that the company might be having issues making a larger Retina Display panel.

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Most experts have said the iPhone 5S will be more of an updated iPhone 5 with minor improvements rather than a completely new device, and will most likely have the same overall display size and design.

If this is the case however, a delay over display issues doesn't add up, meaning the iPhone 5S might be more advanced than originally predicted.

Two key features expected for the iPhone 5S were leaked this week, including a larger battery and camera system that resembles Instagram according to Japanese blog site MacOtakara.

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