Defence Secretary Ben Wallace on Monday said 350 more British troops would be sent to the Polish border, to shore up NATO's eastern flank against any Russian aggression.

Wallace said the troops would strengthen a contingent of 100 British soldiers already there and would be a "bilateral deployment to show that we can work together and send a strong signal that Britain and Poland stand side by side".

The announcement came following talks between Wallace and Polish Defence Minister Mariusz Blaszczak amid growing fears of a Russian invasion of Ukraine, which could spark a massive exodus of refugees into Poland.

Belarus last year allowed migrants to cross its border into Poland, sparking a humanitarian crisis and prompting the deployment of 100 British troops to strengthen the border.

Blaszczak told journalists that the additional British troops would "cooperate with the Polish armed forces on increasing the security, especially when it comes to the threats on the border".

"Based on historical experience, we see that only a decisive deterrence policy can stop any potential Russian aggression and also based on the very same history we do see, the policy of appeasement only encourages potential enemies," Blaszczak added.

Wallace noted that "recently Belarus cynically used human beings as a way of destabilising neighbours". 

"Using illegal migration or indeed forcing people across the border, Britain stood shoulder-to-shoulder with Poland, and sent 100 Royal engineers to support them in those efforts to counter the use of migrants as a tool."

US officials have said the Kremlin has assembled 110,000 troops along the border with pro-Western neighbour Ukraine but intelligence assessments have not determined whether President Vladimir Putin has actually decided to invade.

An invasion could trigger a refugee flood of up to five million people, mainly into Poland, the officials added.

Russia is seeking a guarantee from NATO that Ukraine will not enter the alliance and wants the bloc to withdraw forces from member states in eastern Europe. 

Wallace on Monday insisted that NATO is a defensive alliance and that Russian aggression would be counterproductive and lead to "more NATO, the very thing that President Putin says he doesn't want". 

He said that Russia's "2014 illegal annexation of Crimea led to be enhanced forward presence in Poland, in Estonia, and in Latvia."