Finland, Lithuania, Latvia and Estonia have agreed to coordinate and enhance border measures at the crossings with Russia and Belarus.
ADVERTISEMENTNeighbours of Belarus are reviewing their border security following the Wagner chief Yevgeny Prigozhin's reported deal to be exiled there following the apparent "mutiny" by his mercenaries in Russia.
In Poland, both the Prime Minister and Defence minster inspected troops in the country's north east on Monday.
"We have strengthened the borders - the border with Belarus and the Königsberg region," said Defence Minister Mariusz Blaszczak. "We are aware of these threats and we are responding by anticipating attacks. After all, we have been dealing with a hybrid attack on the Polish border for two years."
Lithuania's President Gitanas Nauseda said on Sunday that NATO would have to "reinforce" its eastern flank if Belarus were to harbour Yevgeny Prigozhin and his troops.
Finland's Foreign Minister Elina Valtonen said the situation remains uncertain: "The situation is delicate and anything can happen. We continue to monitor the situation very closely."
Meanwhile coordinated and enhanced border crossing control measures have been introduced by Finalnd, Estonia, Lithuania and Latvia with Russia and Belarus.
German Defence Minsiter Boris Pistorius announced plans to send around 4,000 additional Bundeswehr permanently to Lithuania to strengthen Nato's eastern flank.
"Germany is prepared to permanently station a robust brigade in Lithuania.brigade in Lithuania on a permanent basis," he said Pistorius during a visit to the Lithuanian capital Vilnius on Monday.
In a turbulent 24 hours in Russia, Saturday saw Prigozhin ordering troops from the private military company (PMC) towards Moscow.
The move briefly threw the immediate future of Russian President Vladimir Putin's leadership into question.
A deal was then struck between Moscow and Wagner, which saw Prigozhin's troops abandon the mission with the Wagner boss going into exile in Belarus.