Apple’s latest gadget the iPhone 4s officially goes on sale in the U.S. and in six other countries on Friday, but stronger than expected demand means that customers who failed to pre-order one of the devices may have difficulty finding any available for purchase.

Apple said it has sold over 1 million units of the iPhone 4S since it went on sale a week ago, just a day after the death of Apple co-founder Steve Jobs stoked massive attention on the company.

Initial reactions to the phone, which was unveiled two days earlier, had been tinged with disappointment since it represents an upgrade to the existing iPhone 4 rather than a completely newly designed iPhone 5.

Though it looks the same as Apple’s current model, it does feature a sharper screen, faster processor and better camera among other key upgrades.

The new device also runs Apple’s latest operating system iOS5 which was launched on Wednesday together with the company’s online file system iCloud. Demand for those services was so strong that it overwhelmed Apple servers causing many customers to get error messages.

In the U.S. the 16-GB model costs $ 199, the 32-GB model costs $ 299, and the 64-GB model costs $399. The phones are also on sale in France, Germany, Japan, the U.K, Australia and Canada.