Apple on Friday said it will "continue to investigate" the unexpected iPhone battery drain as some users still see the problems after downloading a latest software update designed to address the issues.

Apple fans wait outside an Apple store in Hong Kong before the new iPhone 4S goes on sale at 7am on Friday. [CFP]

"The recent iOS software update addressed many of the battery issues that some customers experienced on their iOS 5 devices. We continue to investigate a few remaining issues," said Apple in a statement made to technology news site AllThingsD.

Apple released iOS 5.0.1, an update to its latest mobile operating system iOS 5 on Thursday. The update includes fixes for some bugs which are blamed for shortening the battery life of iPhones and other devices that use the system.

Some users said the battery life of their iPhones look much better after the update. But according to some posts on Apple's official support forums, some iPhone owners still claimed the issue is persisting, saying they saw no difference at all after downloading the update.

Complaints about the battery life of Apple's latest iPhone 4S smartphone have been piling up on the Internet in the past weeks.

On Apple's official support forums, some iPhone 4S owners said their handsets' battery drained more than 10 percent every hour, even when in standby mode with some advanced features like Siri and location services completely deactivated.

It is reported that Apple engineers contacted some iPhone 4S owners in Britain and installed a diagnosis file on their devices to send details back to the company's headquarters in Cupertino, California.

Apple confirmed the iPhone battery issue last Wednesday, saying that a few bugs in the iOS system have caused the problem and it will release a software update in a few weeks.