“The challenge evolves all the time, probably (much faster) than any other type of threat we face at the moment,” said one senior NATO official who asked not to be named.

Newly appointed US Secretary of Defense Chuck Hagel, who will be attending his first NATO meeting since taking office, has made the issue a priority.

In Singapore on Saturday he accused China of waging cyber-espionage against the US after a US report found evidence of a broad Chinese spying campaign against top US defence contractors and government agencies.

“The United States has expressed our concerns about the growing threat of cyber intrusions, some of which appear to be tied to the Chinese government and military,” he told an annual conference known as the Shangri-La Dialogue.

Hagel pressed Beijing to adhere to “international norms of responsible behaviour in cyberspace”, while acknowledging that the establishment of a joint cyber security working group was a positive step in fostering dialogue.

NATO head Anders Fogh Rasmussen made clear earlier this year that cyber-security is a key issue.

The US-led military alliance had been forced to upgrade its defences after several attacks, among them by Anonymous, the hacker group, he said.