"None of our networks are safe," said Devin Nunes, a congressman who heads the intelligence committee in the US House of Representatives. "They are all over everyone's networks."

"The rules of the game have changed", he said, thanks to the hyper-activity of Russian, Chinese and Iranian state actors.

"I am the House intelligence chairman," said Mr Nunes, speaking to reporters before the Australian American Leadership Dialogue in Melbourne. "I'm just telling you that I assume that my email and my phone calls are being listened to."

Asked if everyone was vulnerable, at all times, Mr Nunes said: "Yes. My phone literally is vulnerable."

And asked to make comparisons between the activities of "foreign adversaries" and the US, following revelations by former US intelligence contractor Edward Snowden, Mr Nunes said Mr Snowden had "endangered the lives of American operatives and Australian operatives".

"It should have raised questions for everyone when Snowden fled first to the Chicoms and second to the Russians," he said. "These are the guys who are conducting cyber attacks all over the world and utilising cyber attacks for economic espionage."

Mr Nunes, who has access to data and officials at 17 agencies that make up the vast US intelligence community, was speaking in the wake of what is possibly the largest publicly acknowledged data heist in cyber history.

Officials last month admitted that social security numbers, family details, personal histories and even passwords and finger prints had been stolen from US personnel files containing the results of security background checks for 21.5 million people.

Experts say the use of "big data" analytics on this huge trove of information could yield all manner of sensitive information, including the identity of American spies.

US intelligence officials have said the trail leads back to China, while a Chinese government spokesman has said such claims are "irresponsible and unscientific".

Mr Nunes said Russia and China were clearly the world's worst offenders, not necessarily in that order, followed by Iran. And their targets were not confined to the United States.

"I make the assumption, as the intelligence chair, that when I'm on my mobile phone, or I send an email, that foreign adversaries probably could get a hold of that," he said.

"The rules of the game have changed because you have to be careful about what you want to communicate and what you want our adversaries to pick up."

Australian officials privately blame Chinese hackers for tapping into emails, phones and computers of major Australian corporations, human rights groups and senior politicians right up to the level of prime minister.