CUPERTINO, Calif.: Apple CEO Steve Jobs showed off a new smartphone operating system on Thursday that features an advertising platform to compete with Google's, and revealed stronger-than-expected sales of 450,000 units for the iPad.

The iPhone 4.0 software will be available on Apple's hugely popular smartphone this summer, complete with a number of upgrades, including a long-awaited multi-tasking capability that allows the use of several applications at once.

A version of the iPhone's operating system is also used on the iPad, and the latest generation of software will come to Apple's new tablet computer this fall.

The new advertising platform for the iPhone and iPad -- dubbed iAd -- marks Apple's first foray into a small but growing market, and is sure to please the thousands of application developers who make their living off those devices, providing them with a new revenue stream.

The iPad's early sales impressed analysts, many of whom expect 1 million units to be sold in the quarter ending June, and roughly 5 million in 2010, though estimates vary widely.

"We're making them as fast as we can. Our ramp is going well, but evidently we can't quite make enough of them yet so we're going to have to try harder," Jobs said, noting iPad sellouts at Best Buy stores.

The electronics giant has staked its reputation on the 9.7-inch touchscreen tablet, essentially a cross between a smartphone and a laptop. It is helping foster a market for tablet computers that is expected to grow to as many as 50 million units by 2014, according to analysts.

"I think it's pretty impressive, five days almost half a million units, and it shows there's still pretty good momentum behind the first day," said Gartner analyst Van Baker.

Despite critics who question whether a true need exists for such a gadget, analysts expect Hewlett-Packard, Dell and others to trot out their own competing devices this year.

Since the iPad went on sale on April 3, users have downloaded 600,000 digital books and 3.5 million applications for the device, Jobs said. There are already 3,500 apps available for the iPad.

"It was above my expectations, frankly," said Joe Clark, managing partner of Financial Enhancement Group, referring to iPad sales. "The day the original Apps Store launched it was a game change for the iPhone and it will do the same eventually for the iPad."

At a media event at the company's Cupertino, California, headquarters, Jobs said Apple had so far sold more than 50 million iPhones, the smartphone that competes with Research in Motion's Blackberry and Motorola's Droid.

That implies that the company sold 7 million or more devices in the March quarter, which would be above many analysts' forecasts.