The typically cryptic invitation said "It's almost here", sported a number 12 - corresponding to the date of the event - and cast a large shadow of the number 5, a clue that the fifth version of the popular smartphone could be in the pipeline.

Apple's iPhone launches are among the most-watched events on the tech industry calendar. The latest version of the company's main product -- generating more than half its revenue -- may sport a larger, higher-end screen, sources have said.

The new screen could measure 4 inches from corner to corner, one source has said, an increase from the 3.5-inch display that has been held constant since the smartphone began selling in 2007 and revolutionized the mobile industry.

Some analysts say the larger screen may be a response to rival Samsung Electronics, whose larger, Google Android phones have helped it become the world's biggest smartphone maker.

Speculation had also arisen in past months that the company might offer details about a smaller version of its iPad, but the emailed invitation offered no hint of that on Tuesday.

The event will take place at San Francisco's Yerba Buena Center for the Arts, the preferred venue for the unveiling of past products such as the iPad.

The new iPhone will hit store shelves in time for the crucial holiday season. Shares in the company edged 0.6 percent higher to $669.44.

(Reporting By Alexei Oreskovic; Editing by Tim Dobbyn)

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