Some 140 tech companies, civil liberties and privacy activists urged the White House Tuesday to pull back efforts to weaken encryption or include law enforcement "backdoors" on technology products.

The effort marked the latest turn of events in a dispute between Silicon Valley firms and the US government, which is seeking ways to access encrypted phones and other devices to root out criminals and terrorists.

In a letter to President Barack Obama, the signatories urged the administration "to reject any proposal that US companies deliberately weaken the security of their products."

"Strong encryption is the cornerstone of the modern information economy's security," the letter said.

"Encryption protects billions of people every day against countless threats -- be they street criminals trying to steal our phones and laptops, computer criminals trying to defraud us, corporate spies trying to obtain our companies' most valuable trade secrets, repressive governments trying to stifle dissent, or foreign intelligence agencies trying to compromise our and our allies' most sensitive national security secrets."

The letter was a response to pleas from the FBI and National Security Agency to allow US law enforcement and intelligence services access to encrypted devices as part of lawful investigations.

Those comments in recent months followed moves by Apple, Google and others to enable encryption of phones and other devices, with the keys only in users' hands, so the companies would be unable to unlock or provide data even with a court order.

Tuesday's letter -- endorsed by Facebook, Apple, Microsoft, Twitter, Yahoo and other tech firms -- argued that there is no way to enable this kind of access without weakening security.

"Whether you call them 'front doors' or 'back doors,' introducing intentional vulnerabilities into secure products for the government's use will make those products less secure against other attackers," said the letter.

"Every computer security expert that has spoken publicly on this issue agrees on this point, including the government's own experts."

Kevin Bankston, of the New America Foundation's Open Technology Institute, said the message is important for the White House to hear as it weighs its response on encryption standards.

"We thought it important to ensure that President Obama heard now a clear and unified message from the Internet community: encryption backdoors are bad for privacy, bad for security, bad for human rights, and bad for business," he said.

Others endorsing the letter included the American Civil Liberties Union, American Library Association, Electronic Frontier Foundation, Reporters Committee for Freedom of the Press, the Internet Association and more than two dozen academics and security researchers.