Apple says it has seen no evidence that overheating batteries had caused screens on some of its iPhone devices to explode, blaming incidents in France on "external force."

French authorities have opened an investigation into the safety of the cult smartphone, following claims by several users in France and elsewhere that their iPhone screens had shattered in a dangerous manner during use.

iPhone owners are fast learning that hard surfaces and glass touch screens do not mix.

"To date, there are no confirmed battery overheating incidents for iPhone 3GS and the number of reports we are investigating is in the single digits," the firm said in a statement to AFP.

"The iPhones with broken glass that we have analysed to date show that in all cases the glass cracked due to an external force that was applied to the iPhone," the company added.