There might be another pushback marring the supposedly nearing iPhone 5S release date as Apple reportedly implemented a last-minute production revision - upsizing the phone screen from the 4-inch template to 4.3-inch.

In a report from Taiwan initially picked up by Bloomberg News, it was indicated that the September or October release forecast for Apple's 2013 flagship would be delayed, likely by December.

"Apple may delay the introduction of the iPhone 5S until the end of the year after the design was changed to feature a bigger 4.3-inch Retina display screen," Bloomberg said on its report, citing Taiwan's The Commercial Times as its source.

The tech giant, however, is yet to comment on the news as Bloomberg said that efforts to reach Apple have been unsuccessful as of press time.

Also, the publication attempted to get clarifications from both TSMC and Pegatron, two of Apple's key manufacturing partners, but the two firms declined to issue statements.

The development seemed to belie earlier reports that Apple has already commenced test productions, in small batches, of the iPhone 5S as early as June and is scheduled to begin volume production by the end of July.

And all signals point to a release date that is in line with analysts' projection until the Bloomberg report. Coming from a reputable source, the news somehow carry some weight though MacRumors insist that the claim contradicts the logical production calendar, in which finalising the final handset design is completed months before the actual start of production activities.

If indeed Apple will alter its pre-determined schedules - production and release date of the iPhone 5S - then the chance is high that handset's issuance could come as late as January 2014.

On release date, Apple is expected to deliver an iPhone 5S that is a near-clone of the iPhone 5, with the upgrades noticeable by closer checks on what's inside - likely consisting of a faster A7 processor, Retina+ with 1.5M screen pixels, fingerprint scanner and a host of killer features thanks to iOS 7.