The official unveil of iPhone SE seems imminent on Mar. 21 as the iPhone 5S becomes unavailable in retailers.

Apple had released the iPhone 5S in 2013. It is one of the most popular iPhones ever. Since 4-inch iPhones have been quite popular among iPhone users, Apple is rumored to be working on a new 4-inch iPhone that is dubbed as Apple iPhone SE (Special Edition). 

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The current iPhone 6S and iPhone 6S Plus comes with screen sizes of 4.7-inch and 5.5-inch display, respectively. However, the upcoming iPhone SE is expected to sport a 4-inch screen as well as some of internal specs of iPhone 6S. The U.S. carriers and retailers seem to be exhausting their current stocks of the iPhone 5S, which indicates that they are prepping for the arrival of iPhone SE, 9 to 5 Mac reported.

On Best Buy, several editions of the iPhone 5S through Verizon and Sprint can be found.  However, all of them are mentioned as "Sold Out Online." The unlocked 16 GB and 32 GB iPhone 5S are also unavailable with Best Buy. There are no indications on whether it will be selling more iPhone 5S units in future.

A search for "iPhone 5S" on Target does not yield any results. Moreover, the direct URLs for iPhone 5S and its variants are no longer available. As of this writing, there is no confirmation as to when it will be adding newer stocks of iPhone 5S.

On Walmart, an unlocked edition of iPhone 5S with a plan from Straight Talk is available. Its showing a status of "Rollback" which means that the retailer giant is finishing the existing iPhone 5SS units and it has no plans to replenish its stocks again.

Even U.S. carriers seem to be clearing iPhone 5S stocks. On Verizon, the 32 GB iPhone 5S in colors such as Space Gray and Silver are currently out of stock but are expected to be available in future. The 16 GB iPhone 5S in Space Gray color is available and is expected shipment date is March 18.

The publication has stated that the outside the U.S., the iPhone 5S is expected to continue to sell at reduced rates in countries like India. The iPhone 5S price is expected to be slashed to $225, Mac Rumors reported.

iPhone SE is speculated to be revealed alongside the 9.7-inch iPad and new band options for Apple Watch. Both iPhone SE and 9.7-inch iPad Pro are rumored to be available for buying from March 25.