* International cooperation "at nascent stage" -Napolitano
* "Best minds" scrambling to counter threat
* Lack of legal framework hinders cyber attack response
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VIENNA, July 1 (Reuters) - Cyber criminals are outwitting national and international legal systems that fail to embrace technological advances, a top U.S. official said on Friday, demanding a cross-border campaign to combat the security threat.
"Most countries don't even have a legal framework that really governs cyber. It is such a new phenomenon in that regard so the legal systems -- both domestic and international -- have not kept pace with the technological advances we have seen," U.S. Secretary of Homeland Security Janet Napolitano said.
"And that is just the plain fact of it. We need to accelerate that in response," she told reporters in Vienna.
Prominent cyber attacks of late have targeted the International Monetary Fund, the U.S. Central Intelligence Agency and the U.S. Senate, and companies such as Citigroup and Lockheed Martin Corp .
The attacks have raised questions about the security of government and corporate computer systems and the ability of law enforcement to track down hackers.
"The threats are real. They demand our vigilance, including vigilance in protection of the values we cherish, and they demand continued cooperation," Napolitano said.
Napolitano, in Vienna to address an international security conference, declined to comment on the status of investigations into these cases but stressed countries had to step up their ability to cooperate.
"I would have to say that we are still at the nascent stage. There is no comprehensive international framework" for approaching the issue, she told reporters. The situation was no better in the European Union.
"We are all scrambling but we are scrambling with some of the best minds in the world and we are confident that from a technological point of view we are going to get to a satisfactory resolution of some of these difficult problems.
"Right now there needs to be some sort of international legal framework to address those and that does not yet exist."
A senior Chinese official denied last week there was any cyber warfare between China and the United States, after weeks of friction over accusations that China may have launched a string of Internet hacking attacks.
Accusations against China have centred on an intrusion into the networks of Lockheed Martin and other U.S. military contractors, as well as efforts to gain access to the Google email accounts of U.S. officials and Chinese human rights advocates.
(Editing by Ralph Boulton)