Defence ministers from NATO's 28 member states meet on Tuesday with cyber-defence top of the agenda, amid concerns about the threat posed by increasing cyber-attacks, many blamed on China.

"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.

"We have to make certain that NATO keeps pace with the threat," the official added, looking ahead to the meeting Tuesday and Wednesday.

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.

Early hacker attacks by Serbs during NATO's campaign in Kosovo at the end of the 1990s alerted the alliance to a danger which has only got more serious since then.

In 2008, NATO set up a cyber-defence centre in Tallin, Estonia, one of the most connected countries in the world and which itself had come under attack the previous year.

Estonia accused Russia, NATO's old Cold War foe, of being behind the attacks on its official sites and information networks.

The senior NATO official said the alliance would from this year fix cyber-defence benchmarks for member countries.

The exercise is aimed at protecting information networks, the electronic nervous system at the heart of modern warfare, the offiicial said.

There is no intention to develop "offensive capacities," the official said, adding that of NATO's 28 members, 23 have already signed up to exchange information and help in the event of a cyber-attack.

One diplomat noted that NATO had a special problem because, just as in conventional warfare, some member "states absolutely do not have the same capabilities as others."

Some have minimal defences while others, including the United States, commit major resources to the problem -- but may not always be ready to share their expertise, the diplomat said.

Defence ministers will also discuss Afghanistan, where NATO is progressively handing combat duties over to local forces as it prepares to withdraw in 2014.

A key question is how many troops will be kept in Kabul to run a training and advisory mission post-2014. US officials have mooted a figure of 8,000 to 12,000.

On Friday, a former NATO commander in Afghanistan suggested in a report that the US might need to keep a larger force than initially planned after 2014 to help Afghans build up their military.

Retired four-star general John Allen oversaw NATO operations there for more than a year and a half.