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.