A new battle is brewing over privacy for mobile devices, after moves by Google and Apple to toughen the encryption of their mobile devices sparked complaints from law enforcement.

The issue is part of a long-running debate over whether tech gadgets should have privacy-protecting encryption that makes it difficult for law enforcement to access in time-sensitive investigations.

US FBI director James Comey reignited the issue last week, criticizing Apple and Google for new measures that keep smartphones locked down - without even the company holding the keys to unlock the data.