"When there's a major cyber incident it's very difficult to prove most of the time who did it," said Richard Clarke on Friday, former White House adviser on national security and cyber threats.
"There are incidents, I think, where governments are involved, doing either reconnaissance or testing out concepts, probing for weaknesses."
Clarke said he suspects Russia and China are the most pervasive users of internet for intelligence-gathering on suspected enemy states and plotting ways to use the information for military purposes.
"Maybe the United States too," he said.
Clarke worked for the last three US presidents as a White House national security adviser. He resigned after the September 11 2001 attacks on America and has been a vocal critic of the Bush administration's so-called war on terror and Iraq campaign.
His latest comments come as network security experts report a growing sophistication of attacks on business and government websites, either knocking them offline for long periods or cracking their defences to steal trade secrets.
With more of the world's crucial national infrastructure - from emergency police hotlines to power grids - connected, at times, to the public internet, the risk of cyber attacks is growing.
Clarke resigned after the
September 11 2001 attacks
In addition, a new crop of elaborate computer programmes has been unleashed on the internet, capable of snooping on security networks for top secret information.
Experts agree there is scant evidence so far of state-sponsored efforts to hack into military computer systems or compromise national security networks.
But the growing severity of internet attacks and the rise of malicious spying programmes have led many to conclude this is the handiwork of professionals with advanced computing skills, ample funding and a military mindset.
Law enforcement officials believe organised crime is behind much of the new so-called "spyware" that emerges on the internet daily. The programmes have proved adept at conning consumers out of money or stealing their banking details and major companies have been hit as well.
"Organised hacking is mainly done for economic purposes," said Ira Winkler, a former network security specialist for America's National Security Agency.
He added some governments are also interested in using the medium to steal a march on their economic rivals, as the internet has proved to be one of the best resources for corporate espionage.
"I would hope that one of the lessons we learned from September 11 is that you don't wait for a disaster to occur before we fix the problems we know exist"
Ex-White House security adviser
For that reason, security experts have begun to warn the world's most visible multi-national conglomerates to shore up their networks defences against cyber snooping - with mixed results.
Clarke said the most industrialised nations of the world remain at risk to some form of cyber attacks against both the corporate sector and their national infrastructure because investment in shoring up these networks has been weak.
"I would hope that one of the lessons we learned from September 11 is that you don't wait for a disaster to occur before we fix the problems we know exist," he said.