Apple claims that iPad sales have topped two million in less than 60 days since its launch on 3 April.
The announcement comes just days after Apple began shipping the iPad in the UK, Australia, Canada, France, Germany, Italy, Japan, Spain and Switzerland.
The European release was delayed for a month after unexpectedly high demand in the US. Apple said it sold 500,000 iPads in the US in the first week, which exceeded forecasts of initial sales of between 100,000 and 400,000.
The iPad will be available in nine more countries in July and additional countries only later this year.
"We appreciate their patience, and are working hard to build enough iPads for everyone," said Steve Jobs, chief executive at Apple.
Apple said developers have already created more than 5,000 applications for the iPad that take advantage of its Multi-Touch user interface and high-quality graphics.
The iPad will also run almost all of the more than 200,000 apps on the App Store, including apps already purchased for the iPhone or iPod touch, Apple said.