Apple said on Tuesday it would host its fall event on September 12, setting the stage for what analysts believe will be the unveiling of a new line of iPhones and smartwatches.

The event will be hosted at the Steve Jobs Theater at its headquarters in Cupertino, California, according to invites from the world's most valuable company.

Wall Street analysts have said Apple will try to entice shoppers with a range of new features for its flagship device, as the launch comes against the backdrop of a slump in smartphone demand globally.

Apple posted a 2.4 per cent decline in iPhone sales for its fiscal third quarter - a rare drop for the product that has for years powered the company's growth.

The most expensive variant of the new generation iPhone will have a periscope camera that could improve zoom capacity by 5 times or more, according TF International Securities analyst Ming-Chi Kuo.

The expected watch lineup may feature a new processor based on Apple's A15 Bionic chip, already used in previous iPhone models, and will boost performance, according to a Bloomberg News report.