Russian submarines could cut off communications cables between the US and the UK, a defence chief has warned.
Sir Stuart Peach, chief of the British defence staff, said Russian ships have been spotted close to Atlantic cables which are essential for international commerce and the internet.
A report by think tank Policy Exchange estimated earlier this month that $10tn is transmitted in transactions through the cables.
Speaking at think thank RUSI in London, Peach said Russia has developed "unconventional" tactics for warfare. Russian forces have a precedent for severing communications cables, after it cut Crimea off from the outside world in order to annexe it.
Peach is set to become the most senior Nato military officer next year when he takes up the post of chairman of its military committee.
He warned the UK and its allies should develop maritime forces in order to match Russian fleets.
He said: "It is essential to our security that we sustain our posture as Russia modernises its forces and flexes its military muscles with a higher risk appetite to achieve its national interest."