SAN FRANCISCO – Apple Inc distributed invitations to an event in San Francisco on Sept. 12, setting the stage for what is widely expected to be the release of the iPhone 5.

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.