Phone production has not been smooth, as supply chain sources told Reuters the backlight that helps illuminate the screen had to be revised, putting screen assembly on hold for about a month.
Speculation about the use of sapphire glass in Apple's screens has also risen with the firm's acquisition of a production plant in Arizona and strong demand for tougher screens.
Wired Magazine also reported that the new phone will feature a payment feature, likely built upon near field communication technology.
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The Cupertino-based firm is also reportedly producing a 12.9-inch iPad, although analysts were not sure if the new iPad would come out in September or during a separate event, thought to occur in October.
Apple's push for bigger displays is thought to be an effort to counter the success of archrival Samsung Electronics with its larger devices, especially in Asia.
Samsung has stepped up the competition with Thursday's unveiling of its Gear S smartwatch and the launch of its latest smartphone, the Galaxy Alpha, earlier this month.
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"We don't know if there will be an iWatch at the event," Forrester analyst J.P. Gownder said. But "Apple has a long history of of taking newish products and turning them into a strong category."
—By CNBC's Evelyn Cheng. Re/code and Reuters contributed to this report.
CNBC's parent NBC Universal is an investor in Re/code's parent Revere Digital, and the companies have a content-sharing arrangement.