Microsoft Corp's new Surface tablet - its challenger to Apple's iPad - had a "modest" start to sales because of limited availability, Microsoft Chief Executive Steve Ballmer told French daily Le Parisien.

The world's largest software company put the Surface tablet center stage at its Windows 8 launch event last month in its fightback against Apple and Google in the exploding mobile computing market.

"We've had a modest start because Surface is only available on our online retail sites and a few Microsoft stores in the United States," Ballmer was quoted as saying.

Meanwhile, 4 million upgrades to Windows 8 were sold in the three days following the system's launch, Ballmer added. (Reporting by Lionel Laurent; Editing by David Cowell)