Apple has sold 450,000 iPads since the US launch of the device; with the company already experiencing stock problems due to high demand.

Speaking at an Apple press event, at which the iPhone OS 4 was also previewed, CEO Steve Jobs admitted that the company was finding it difficult to keep up with demand.

The $499 device sold 300,000 units in its first day, including pre-orders, with 1 million apps downloaded in the first 24 hours and 3.5 million in total so far.

"Our [manufacturing] ramp is going well, but evidently we can't make enough of them yet," said Jobs - as quoted by website Gamasutra.

An alternative version of the iPad hardware featuring 3G compatibility is expected to be released in the U.S. later this month. The basic wi-fi enabled iPad is also due for release in Europe in April.

"It feels great to have the iPad launched into the world—it’s going to be a game changer," said Jobs following the iPad's initial release. "iPad users, on average, downloaded more than three apps and close to one book within hours of unpacking their new iPad."