Top U.S. tech companies are warning the Obama administration against imposing new policies that the companies say would weaken increasingly sophisticated encryption systems designed to protect consumers' privacy.

In a strongly worded letter to President Barack Obama on Monday, two industry associations representing major software and hardware companies said, "We are opposed to any policy actions or measures that would undermine encryption as an available and effective tool."

The Information Technology Industry Council and the Software and Information Industry Association, representing tech giants, including Apple Inc, Google Inc, Facebook Inc, IBM and Microsoft Corp, fired the latest salvo in what could be a long fight over government access into smart phones and other digital devices.

Obama administration officials have pushed the companies to find ways to let law enforcement bypass encryption to investigate illegal activities including terrorism threats, but not weaken it in a way that would let criminals and computer hackers penetrate the security wall.

So far, however, the White House has not spelled out specific regulatory or legislative steps that it might seek to achieve that objective.

Last week White House press secretary Josh Earnest called this a "thorny policy challenge" that has Obama's attention.

While he recognized tech companies' efforts to protect Americans' civil liberties, Earnest, responding to a reporter's question, added that the companies "would not want to be in a position in which their technology is being deployed to aid and abet somebody who’s planning to carry out an act of violence."

The industry letter to Obama also was sent to FBI Director James Comey, Homeland Security Secretary Jeh Johnson, Attorney General Loretta Lynch and other Cabinet heads.

Days earlier, the United States enacted legislation that will curtail the government's ability to scoop up huge volumes of data related to records of Americans' telephone calls.

The government surveillance was an outgrowth of the Sept. 11, 2001, attacks on the United States and was exposed by former National Security Agency contractor Edward Snowden.

The industry groups noted that online commerce has flourished in part because consumers believed their payment information would be secure.

"Consumer trust in digital products and services is an essential component enabling continued economic growth of the online marketplace," the industry wrote.

"Accordingly, we urge you not to pursue any policy or proposal that would require or encourage companies to weaken these technologies, including the weakening of encryption or creating encryption 'work-arounds'."