India, Oct. 13 -- NATO's top military commander said the alliance is "ready to fight tonight" if it is attacked, as European officials warn Russia could test NATO's defences with increased covert attacks or a limited military incursion.

European nations have in recent months faced an escalation of covert Russian activity, including an arson attack on an Estonian defence company and a failed attempt to attack a Ukrainian aircraft at a German airport.

If Russia decides to attack NATO, the alliance will respond by using sensors, autonomous systems and military force to stop any assault, NATO's Supreme Allied Commander US General Alexus Grynkewich said in Warsaw on Monday.

Grynkewich spoke at the Warsaw Security Forum to set out what he ca...