Apple today said it will release a software update to its iOS 5 operating system, acknowledging some customer complaints on the performance of the new iPhone 4S battery.

"A small number of customers have reported lower-than-expected battery life on iOS 5 devices," Apple spokesman Natalie Harrison said.

"We have found a few bugs that are affecting battery life and we will release a software update to address those in a few weeks."

Apple did not provide any more details but a rash of complaints on the new phone's short battery life have cropped up in online forums.

Ms Harrison declined to comment further about what may be causing the problem.

The statement comes after users took to message boards and discussion forums to complain about weak battery life after downloading the new iOS 5 mobile operating software that was released last month with the iPhone 4S.

The glitch is the second consecutive iPhone release in which Apple has had to issue a fix. Last year, Apple released a software update and gave away free cases after some customers complained that the handset would drop calls if gripped a certain way.

"I am experiencing very very poor battery life on iPhone 4 since installing iOS 5," said one person on Apple's discussion forum. "I upgraded to iOS 5 last night and I noticed that my battery only lasts about 5 hours from full charge," said another. The battery complaints have not harmed Apple's stock price, which rose 90 cents to $397.41 in US trading yesterday.

The shares have gained 23 per cent this year. The company sold more than 4 million of the new iPhone 4S in its opening weekend last month