San Francisco, Apr 9 (DPA): Apple announced a new operating system for its market-leading iPhone Thursday, as it seeks to thwart challenges to its smartphone supremacy from rivals like Google, Microsoft, Nokia and the Blackberry.

The new software was announced just days after Apple made headlines with the launch of its iPad tablet computer, and amid reports that the company was planning a smaller version of the device to help boost sales.

Apple CEO Steve Jobs said that the company had sold 450,000 iPads since they went on sale Saturday. At the same time users had downloaded 3.5 million apps and over 600,000 books, Jobs said.

The new iPhone code, which will be released in the summer, will help dispel the most frequent criticism of the iPhone: that it does not allow multi-tasking - the ability to run more than one application to run at a time.

"We weren't the first to this party but we're going to be the best," said Jobs. "It's really easy to implement multitasking in a way that drains battery life. If you don't do it just right your phone's going to feel sluggish and your battery life is going to go way down.

"We've figured out how to implement multitasking of third-party apps and avoid those things."

Apple also took a clear stab at Google's hopes to dominate mobile advertising in the same way that it dominates search advertising on laptops and desktops.

Jobs announced the launch of iAds, a function that will allow advertisers to buy targeted interactive ads that will feature inside the apps that users activate on their iPhones. It could also reap billions of dollars of new income for Apple, which

will take 40 percent of ad revenues, with the other 60 percent going to the developers of the apps.

The iPhone 4.0 will also be optimized for Voice-Over-IP services like Skype, so that users can make calls without using their cellular minutes, and also receive VOIP calls when their devices are in sleep mode.

Other new capabilities include a unified inbox in which all incoming communications can be accessed, including SMS, voicemails and emails from different accounts. Apple's newly launched iBooks online bookstore will also become available for the iPhone, with the ability to synchronize page and bookmarks between devices.

Apple also announced the launch of a social gaming network for the iPhone, claiming the device had more than ten times the number of games available for traditional hand-held consoles like the Nintendo DS and Sony's PSP.

"iPhone OS 4 is the fourth major release of the world's most advanced mobile operating system," said Jobs. "We're delivering over 100 new features, including multitasking, folders, a unified inbox, deeper Enterprise support, and an iPhone version of our iBooks reader and online iBookstore."