As few as 12 different Chinese groups, largely backed or directed by the government there, do the bulk of the China-based cyberattacks stealing critical data from US companies and government agencies, according to US cybersecurity analysts and experts.

The aggressive, but stealthy attacks, which steal billions of dollars in intellectual property and data, often carry distinct signatures allowing US officials to link them to certain hacker teams. And analysts say the US often gives the attackers unique names or numbers, and at times can tell where the hackers are and even who they may be.

Sketched out by analysts who have worked with US companies and the government on computer intrusions, the details illuminate recent claims by American intelligence officials about the escalating cyber threat emanating from China. And the widening expanse of targets, coupled with the expensive and sensitive technologies they are losing, is putting increased pressure on the US to take a much harder stand against the communist giant.

It is largely impossible for the US to prosecute hackers in China, since it requires reciprocal agreements between the two countries, and it is always difficult to provide ironclad proof that the hacking came from specific people.

Several analysts described the Chinese attacks, speaking on condition of anonymity because of the sensitivity of the investigations and to protect the privacy of clients. China has routinely rejected allegations of cyberspying and says it also is a target.

"Industry is already feeling that they are at war," said James Cartwright, a retired Marine general and former vice chairman of the Joint Chiefs of Staff.

A recognized expert on cyber issues, Cartwright has come out strongly in favor of increased US efforts to hold China and other countries accountable for the cyberattacks that come from within their borders.

"Right now we have the worst of worlds," said Cartwright.

"If you want to attack me you can do it all you want, because I can't do anything about it. It's risk free, and you're willing to take almost any risk to come after me."

The US, he said, "needs to say, if you come after me, I'm going to find you, I'm going to do something about it. It will be proportional, but I'm going to do something ... and if you're hiding in a third country, I'm going to tell that country you're there, if they don't stop you from doing it, I'm going to come and get you."