NATO must build a "cyber shield" to protect the transatlantic alliance from any Internet threats to its military and economic infrastructures, a top US defence official said Wednesday.

Cyber security is a "critical element" for the 28-nation alliance to embrace at its summit of leaders in Lisbon on November 19-20, US Deputy Defence Secretary William Lynn said in Brussels.

"The alliance has a crucial role to play in extending a blanket of security over our networks," Lynn said.

"NATO has a nuclear shield, it is building a stronger and stronger defence shield, it needs a cyber shield as well," he said at a forum hosted by the Security & Defence Agenda think-tank.

The Pentagon's number two called for adopting the Cold War-era strategy of "collective defence" in the cyber arena.

"The Cold War concepts of shared warning apply in the 21st century to cyber security. Just as our air defences, our missile defences have been linked so too do our cyber defences need to be linked as well," Lynn said.

The US government estimates that more than 100 foreign intelligence agencies or governments try to hack into US systems "on a daily basis," he said, highlighting the magnitude of the challenge.

"I think they see the asymetric advantage that can be gained through cyber technology," Lynn said.

The threat of cyber attacks was highlighted in Estonia, a NATO member, in 2007 when it suffered an assault that paralysed key business and government web services for days.

The Pentagon was forced to review its own digital security in 2008 after the most serious cyber attack on the US military's networks, which came from a tainted flash drive that was inserted in a military laptop in the Middle East.

Lynn said the Pentagon strategy has identified "five pillars" to cyber security: recognising cyberspace as the next domain of warfare; the need for active defences; the protection of critical infrastructure; enhancing collective defence; and the need to "marshall our technological prowess."

Lynn stressed that any cyber security strategy needs to take into account threats to critical infrastructure for economies such as power grids, transport systems and financial markets.

"NATO indeed needs to take decisive action to defend its networks," he said.

"I think at Lisbon we will see the kind of high-level leadership commitment to cyber defence. It's the foundation for any alliance effort," he said.

Lynn said he discussed cyber security at a meeting with NATO's decision-making body, the North Atlantic Council, in Brussels on Wednesday.

"I was very impressed with the unity of purpose and the similar vision that most nations in the alliance seem to have towards the cyber threat," he said.