Google said it has turned to encryption to defeat unrelenting online spying by the National Security Agency (NSA) and other U.S. intelligence services.

The countermeasure was announced by Google following recent revelations by fugitive whistleblower Edward Snowden about more sophisticated hacking by US spy agencies. On March 20. Gmail implemented an encrypted HTTPS connection that blocks spying when users check or send email.

Encryption generally prevents outsiders from intercepting a person's messages or documents. Experts noted that Google's encryption will make it very difficult for the NSA or others to tap into email traffic directly.

On the other hand, they said that persistent attacks such as those the NSA is capable of can gain access through malware or other methods that trick users into revealing their passwords.

Google has already begun scrambling most of the traffic at its websites along with technology firms wanting to thwart U.S. intelligence agencies that continue spying on what people are doing online despite worldwide public condemnation.

Similar countermeasures have been announced by Yahoo, Microsoft and Facebook, which are also using encryption that limits the ability of a third party to read messages or emails.

"Today's change means that no one can listen in on your messages as they go back and forth between you and Gmail's servers—no matter if you're using public WiFi or logging in from your computer, phone or tablet," said Gmail engineering security chief Nicolas Lidzborski in a blog post.

"Your email is important to you, and making sure it stays safe and always available is important to us."

Lidzborski said Google's latest move "ensures that your messages are safe not only when they move between you and Gmail's servers, but also as they move between Google's data centers, something we made a top priority after last summer's revelations."