Fallon said that NATO must be prepared for Russian President Vladimir Putin to threaten former Soviet bloc countries including Latvia, Lithuania or Estonia.
The comments made to reporters during a trip to Sierra Leone were among the harshest made by senior British officials in recent months.
The Ministry of Defence on Thursday said that Royal Air Force jets had escorted Russian bombers away from British airspace after they approached on Wednesday. There has been an increase in Russian probing of British defenses recently.

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