Nato defence ministers are set to give the go-ahead this week to a new multi-national force of 4,000 troops to bolster the defence of Poland and the Baltic states.
The United States, Canada, Germany and Britain are set to lead battalions of 1,000 troops each.
Last month Russia's Defence Minister Sergei Shoigu urged Nato not to go ahead with the move and warned that Moscow would respond by posting three new divisions of its own close to the frontier.
Russia's annexation of the Crimea in 2014 and its support for ethnic Russian rebels in eastern Ukraine has made Poland, Lithuania, Latvia and Estonia increasingly anxious about their own security.
Asked about the possibility of an Russian attack, Lithuania's Defence Minister Juozas Olekas told Reuters: 'We cannot exclude it...They might exercise on the borders and then switch to invasion in hours.'
Lithuania, Latvia and Estonia say they are putting their armies on a war footing, meaning they can be mobilised almost immediately.
Baltic on war footing: Nato is to send four multi-national battalions to Poland, Lith
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