SAN FRANCISCO, USA: Apple Inc has sold 3 million iPads in less than three months, a faster-than-expected pace that boosted its stock days before the company's newest iPhone hits store shelves.

Several brokerages, including Kaufman Bros and UBS, raised their estimates on Apple's earnings or revenue on Tuesday. Shares in the company, which starts selling the iPhone 4 in the five of the world's six largest economies on Thursday, shot up as much as 2.1 percent before ending the day up 1.4 percent.

Also read: iPad hits Japan in international launch

Investors will now turn their attention to Thursday's iPhone launch. Apple moved 600,000 of the slimmer and better-equipped, $199 iPhone 4 in its first day of pre-sales, in a deluge of interest that jammed up ordering systems and squeezed carrier partner AT&T's inventory.

Analysts expect the usual lines of people at stores come Thursday, hoping to snag one of the "limited" quantity slated for retail sale only. AT&T will begin selling the iPhone 4 at retail on June 29, on a first-come first-served basis.

Barclays Capital estimated on Tuesday that Apple will move 8.1 million iPhones in the fiscal third quarter ending June 26, surging to 12.1 million in the fiscal fourth quarter as the iPhone 4 production chain accelerates to meet demand.

JP Morgan estimated that 10 million iPhones will ship every quarter as the iPhone 4 takes hold, from around 8 million in the current quarter.

"Some in the investment community may not grasp the magnitude of these initial iPhone 4 shortages," said Barclays analyst Ben Reitzes in a research note. "Supply is likely to be the limiting issue for Q3. However, we believe Apple's production ramp for the September and December quarters is very significant."

The iPhone is Apple's main growth driver at the moment, and is expected to soon become its biggest source of revenue as the company expands the smartphone around the globe.

The iPhone 4 begins selling in the U.S., Japan, France, Germany and Britain this week but will be in 88 countries by September -- the company's fastest-ever global product roll-out.

Early indications are that the smartphone -- which sports a higher-resolution screen, about 40 percent more battery life, video chat via Wi-Fi, and a gyroscope sensor for improved gaming -- will be well-received.

Tech columnist Walt Mossberg, who tested the gadget for a week, raved about the simple-to-use video chat and about four-times sharper display, but warned that Apple had not yet fully addressed longstanding complaints about the iPhone's inability to multi-task, or run applications simulatneously. The device now offers limited multi-tasking.

"In both hardware and software, it is a major leap over its already-excellent predecssor, the iPhone 3GS," Mossberg wrote in the Wall Street Journal.

Strong demand

Those currently ordering the new iPhone via Apple's online store will have their devices shipped by July 14, according to the company's website.

The site says "limited" iPhone 4 quantities will be available in Apple retail stores on Thursday for those who didn't preorder the smartphone.

As investors and consumers await the market debut of the newest-generation iPhone, investors are increasingly convinced that the iPad is already a hit. The tablet's faster-than-anticipated sales have helped Apple overtake Microsoft Corp to become the world's most valuable technology company, .

"We as well as the Street continue to underestimate demand for iPad. Part of the reason is that the iPad, as a tablet, is a relatively new product category where it fits in between a smart phone and a notebook PC," Kaufman Bros analyst Shaw Wu wrote in a research note.

Before the launch in early April many on Wall Street had expected the company to sell roughly one million iPads in Apple's current quarter.

Wu raised his fiscal 2010 earnings and revenue estimates for Apple and hiked his price target by $2 to $342. He now expects Apple to sell 9.7 million iPads for calendar 2010.

"Improved supply and international expansion has helped Apple accelerate the rate at which it is selling the device," UBS analyst Maynard Um wrote in a note.

He estimated that every 100,000 iPads sold adds one penny to earnings per share.

Apple sold 1 million iPads in the first month after the U.S. launch on April 3, and had sold 2 million iPads by May 31. It hit the 3 million mark on June 21, it said in a release on Tuesday.

In addition to the United States, the iPad is currently available in Australia, Canada, France, Germany, Italy and Japan, among other countries. It will be rolled out in nine more markets in July.

Shares of Cupertino, California-based Apple closed up $3.68, or 1.36 percent, to $273.85.