Nato countries must spend money bolstering their armed forces to counter the threat posed by Russia - and risk turning back the clock to the days of the Cold War, according to the alliance’s Secretary-General.

Anders Fogh Rasmussen invoked the spirit of Alexandre Dumas’ The Three Musketeers as he urged Nato’s members to make their motto “all for one, one for all” in the face Moscow’s “illegal aggression” in Ukraine.

In Ukraine, there was growing concern that Russia could intervene in the east of the country following its annexation of Crimea. Pro-Moscow crowds seized government buildings in three cities on Sunday and called for the region to succeed and join Russia.

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Mr Rasmussen, writing in The Daily Telegraph, said: “Today’s crisis shows that defence matters as much as ever. So every ally needs to invest the necessary resources in the right capabilities.

“That means modern equipment, intensive training for our forces, and closer cooperation among Nato allies and with our partners. I know how challenging this is in today’s economic climate, but the security climate makes it vital.

“In the long run, a lack of security would be more costly than investing now and we owe it to our forces, and to broader society. We all benefit from Nato’s protection; we all must be able to contribute to it. The alliance has kept us safe for 65 years. The bond between the democracies of North America and Europe remains the bedrock of our collective security.”

The world is still a dangerous place with new threats such as cyber attacks, but others were age-old -- “attempts to redraw borders by force”.

“What has not changed is Nato’s commitment to our fundamental values and purpose. Our motto remains: all for one, one for all,” Mr Rasmussen said.

“Nobody in Nato wants a return to the Cold War, but we see the Kremlin trying to turn back the clock and carve up Europe into new spheres of influence.

“We must stand up for our values, on which we have built a new and better Europe, and for the system of international rules that has underpinned prosperity and predictability.”

Nato’s actual motto is “Animus in Consulendo Liber”, which is difficult to translate into English. Nato’s website suggests “in discussion, a free mind” or “Man's mind ranges unrestrained in counsel”.