Secretary of State John Kerry says he writes emails "with the awareness" that Russian and Chinese hackers are likely reading them, he revealed in an interview with CBS News on Tuesday.

"It is very likely. It is not ... outside the realm of possibility and we know they have attacked a number of American interests over the course of the last few days," Kerry said, referring to recent reports that Russian hackers attacked the Pentagon and Chinese cyberspies were reading US officials' emails.

"It's very possible ... and I certainly write things with that awareness," Kerry added. He did not specify whether he was referring to personal email, government email, or both.

Chinese cyberspies have reportedly been reading the private emails of Obama-administration officials and "top national security and trade officials" since 2010.

The report comes amid the ongoing issue of Hillary Clinton's use of a private email server for work-related correspondences while she served as Secretary of State from 2009 to 2013.

Her private server was handed over to the FBI yesterday as part of an ongoing investigation.

Personal inboxes are not the only things being targeted: Chinese hackers — known as the "bullies of cyberspace" — have breached airlines, health-insurance companies, and other government agencies to collect intelligence on US officials and their foreign contacts.

More than 21 million people had their sensitive background and security-clearance information stolen when Chinese hackers breached Office of Personnel Management (OPM) databases in early 2014. The same hackers reportedly attacked United Airlines — the main airline flying in and out of Washington, DC's Dulles Airport — and American airlines.

Kerry emphasized that he is well aware of the changing nature of espionage. He also noted that cyberattacks are of "enormous concern" to the administration.

"Spying has taken place for centuries and the latest means of spying is to be going after peoples' cyber," he said. "Companies spend billions of dollars to protect themselves, the United States government does the same."

Still, as geopolitical expert Ian Bremmer noted to Business Insider in June, "there's no effective defense against these attacks and, as we've seen, there's also no effective deterrence."