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.

"What concerns me about this is companies marketing something expressly to allow people to place themselves beyond the law," the FBI chief said, warning that law enforcement may be denied timely access, even with a warrant, in cases ranging from child kidnapping to terrorism.

Former FBI criminal division chief Ronald Hosko made a similar point in the Washington Post, citing a case in which the agency used smartphone data to solve a brutal kidnapping just in time to save the life of the victim.

"Most investigations don't rely solely on information from one source, even a smartphone," he said. "But without each important piece of the investigative puzzle, criminals and those who plan acts destructive to our national security may walk free."

Observers who follow privacy and encryption say they have seen this debate before.

In the mid-1990s, as the Internet was gaining traction, the government pressed for access to digital "keys" to any encryption software or hardware, before abandoning what ended up being a futile effort.

"This is Crypto Wars 2.0," says Joseph Hall of the Center for Democracy and Technology, a digital rights group active in both campaigns.

Today, "the main difference is that phones are increasingly deeply personal, containing much more daily life and interaction than a desktop from the 1990s," Hall said.

Hall argued that giving law enforcement access requires companies to "engineer vulnerabilities" which could be exploited by hackers.

"There's no way to tell the difference between a good guy and bad guy when they walk through the back door," he said.

In 2013, before the revelations of massive surveillance from leaked National Security Agency documents, the FBI called for broader authority to capture mobile communications that fall outside traditional surveillance, such as Skype and Google Hangouts.

But civil liberties activities say leaked NSA documents suggest that contrary to FBI claims made last year, the government has many tools at its disposal.

"There are an increasing number of places where we leave our digital trails," Hall said, including in the Internet cloud, which can be accessed with a court order.

Mike Janke, chief executive of the firm Silent Circle, which makes the fully encrypted Blackphone, said a small number of people may use encryption for nefarious purposes. "Do you sacrifice the privacy and trade secrets of everyone else because of that?" he asked.