SAN FRANCISCO - Apple announced on Wednesday that iPhone 4S, its latest smartphone product, will be available in China and 21 other countries as of January 13.

"Customers response to our products in China has been off the charts," Tim Cook, Apple's chief executive officer, said in a statement.

He noted that with the launch in China next week, iPhone 4S will be available in over 90 countries and regions, making it Apple's fastest iPhone rollout ever.

The iPhone 4S was launched in October last year.

Apple is scheduled to report its 2012 first fiscal quarter earnings on January 24. As one of Apple's biggest revenue sources, the iPhone 4S sales are seen key to the results. In the last quarter, Apple missed projections from the Wall Street as customers had been awaiting the launch of iPhone 4S.

Last October, Apple announced that it had sold more than four million units of iPhone 4S, just three days after the launch. Analysts have been giving rosy estimates on iPhone 4S sales for the first quarter after it hit the stores, saying that the company could have sold more than 30 million iPhones during the period.

The iPhone 4S, which looks the same as the iPhone 4, has major hardware upgrade inside, including a faster chip, a higher- resolution camera and Siri intelligent voice control.