The spokesman would not comment directly on a Washington Post report that said Chinese websites are being heavily used to break into unclassified US government computer networks.

"When it comes to Defense Department networks, ... we have a very aggressive effort to defend our networks," said the spokesman, Bryan Whitman. "We get probed hundreds of times a day, but most importantly, there have been no reports to date that suggest any of our classified networks have been penetrated."

The Defense Department has some 12,000 local area networks with 3.5 million individual computers, he said.

All available tools are being used to blunt computer network attacks and identify their source, he said.

"But all networks have vulnerabilities," he noted, "and the challenge that we have is to stay one step ahead of our adversaries."

US officials told the Post they were unsure if the Chinese government is involved in the attacks on US government computer networks.

"Is this an orchestrated campaign by PRC (the People's Republic of China) or just a bunch of disconnected hackers? We just can't say at this point," said an official quoted by the Post.

Some Pentagon experts, said the daily, are convinced of official Chinese involvement, while others see the electronic probing as the work of other hackers simply using Chinese networks to disguise the origin of the attacks.

Among the computer networks affected were those of the Departments of Defense, State, Energy and Homeland Security, the Post said.

Although classified systems have not been compromised, US officials are concerned that information pulled from various sources could yield useful intelligence to an adversary, the Post said.

"The scope of this thing is surprisingly big" and stretches back as far as two or three years, said one official.

Another official said the Federal Bureau of Investigation has opened an investigation into the incidents, which have been code-named Titan Rain. The daily said the FBI declined to comment on the matter.

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