In its latest PR move for the iPhone 4S, Apple announced today that more than 4 million units of its newest handset sold in the first three days after the device officially went on sale in the US and six international markets on Friday, October 14. The brisk sales of the iPhone 4S are more than double that of the iPhone 4, and break all previous iPhone sales records.

On top of sales of the iPhone 4S, the Cupertino, California-based electronics company also said that more than 25 million devices are now running iOS 5, Apple’s newest mobile operating system, which works on iPhone 4S, 4 and 3GS, iPad and iPad 2, and the third- and fourth-generation iPod touch. IOS 5 was released last Wednesday, two days before the iPhone 4S went on sale at a variety of retail locations. In addition, Apple says 20 million people are now signed-up for its new iCloud service, which allows users to sync content across all of their iDevices automatically.

“iPhone 4S is off to a great start with more than four million sold in its first weekend—the most ever for a phone and more than double the iPhone 4 launch during its first three days,” said Apple senior VP of product marketing Philip Schiller in a prepared statement. “iPhone 4S is a hit with customers around the world, and together with iOS 5 and iCloud, is the best iPhone ever.”

Apple is pushing hard this year to get word out that the iPhone 4S isn’t the dud some believed it would be. Earlier, the company announced that it sold more than 1 million iPhone 4S units during the first 24 hours after pre-orders for the device became available.

In the US, the iPhone 4S costs between $199 and $399, depending on size of internal storage, with the signing of a two-year contract with either AT&T, Verizon Wireless or Sprint, which was just added this year to the list of iPhone carriers. The iPhone 4S is currently available in the US, Canada, German, France, Japan, Australia and the UK. On October 28, the coveted handset will go on sale in an additional 22 countries, including Austria, Belgium, the Czech Republic, Denmark, Estonia, Finland, Hungary, Ireland, Italy, Mexico, the Netherlands, Norway, Singapore, Slovakia, Slovenia, Spain, Sweden and Switzerland. More than 70 countries will have the iPhone 4S by the end of 2011.

The iPhone 4S, which looks identical to last year’s iPhone 4, features a faster, dual-core processor, better 8-megapixel camera, and the highly-acclaimed Siri voice recognition assistant.

Update: Additional contextual details added at 11:45am ET