The new operating system for Apple's iPhone will allow multi-tasking, letting users run more than one program at a time, Apple chief executive Steve Jobs said Thursday.

"We weren't the first to this party but we're going to be the best," Jobs said at an event at Apple headquarters here to unveil version 4.0 of the iPhone operating system.

Jobs said iPhone OS 4.0 includes over 100 new features, chief among them multi-tasking, which devotees of the smartphone have been clamoring for.

Explaining the delay in bringing multi-tasking to the iPhone, Jobs said Apple was concerned about slowing down other features or quickly draining the battery.

Apple has figured out "how to avoid those things," he said.

Other new features include the ability to create playlists on the phone, the addition of a 5X digital zoom to the camera and the ability to use Bluetooth keyboards.

A spell check function was also added to the phone and users now can buy an application and gift it to someone directly through their iPhone.

Jobs said that Apple has sold more than 50 million iPhones to date. If you include iPod Touches, the number shoots up to 85 million, he said.

Jobs also released the latest sales figures for the iPad tablet computer.

He said Apple has sold 450,000 iPads as of Thursday and 600,000 electronic books have been downloaded for the device.

"We're making them as fast as we can," Jobs said. "Evidently we can't make enough of them yet so we are going to have to try harder."

The Apple chief executive also said that iPad owners have downloaded 3.5 million applications for the iPad as of Thursday.

Apple sold over 300,000 iPads on Saturday, its first day of availability in the United States.