NATO leaders have agreed that a large-scale cyber attack on a member country could be considered an attack on the entire US-led alliance, potentially triggering a military response.
The decision marks an expansion of the organisation's remit, reflecting new threats that can disable critical infrastructure, financial systems and government without firing a shot.
"Today we declare that cyber defence is part of NATO's core task of collective defence," NATO Secretary-General Anders Fogh Rasmussen told a news conference.
In 2007, a series of crippling cyber attacks paralysed much of Estonia in an apparent response to a dispute over the movement of a Soviet-era war memorial. Most Western experts suspected the Kremlin was responsible but Russia denied it.