Apple announced on Monday that it sold more than five million units of its latest smartphone product, the iPhone 5, in just three days after last Friday's launch, but the number still missed some analysts' estimates.

"Demand for iPhone 5 has been incredible and we are working hard to get an iPhone 5 into the hands of every customer who wants one as quickly as possible," said Tim Cook, Apple's chief executive officer, in a statement.

In the same period last year, Apple sold four million units of the previous iPhone 4S.

Meanwhile, the company said that more than 100 million iOS devices have been updated with iOS 6, the latest version of Apple's mobile operating system.

The initial iPhone5 sales number fell short of some analysts' estimates, which ranged from 6 million to 10 million. According to some analysts, supply constraints may be the main reason why Apple has reported the "disappointing" number.

"We find it unfortunate that some analysts continue to publish irresponsible estimates without taking into account realistic demand trends and potential supply constraints on new in-cell touchscreens," Shaw Wu, a well-known analyst from Sterne Agee, told Fortune.

According to AppleInsider, Morgan Stanley said in a note to investors that Apple could sell 50 million units of the iPhone from October through December, and that the company's share price could rise to as high as $960 under a "bull case" scenario.