President Vladimir Putin on Friday accused NATO member Poland of having territorial ambitions in the former Soviet Union, and said any aggression against Russia's neighbour and close ally Belarus would be considered an attack on Russia.
Moscow would react to any aggression against Belarus, which forms a loose "Union State" with Russia, "with all the means at our disposal", Putin told a meeting of his Security Council in televised remarks.
Warsaw's Security Committee decided on Wednesday to move military units to eastern Poland after members of the Russian Wagner mercenary force arrived in Belarus, the state-run news agency PAP quoted its secretary as saying on Friday.
Poland denies any territorial ambitions in Belarus.
In his remarks Putin had also stated that the western part of Poland was a gift from Soviet leader Joseph Stalin to the country and that Russia would remind Poles about it.
In apparent reference to that, Polish Prime Minister Mateusz Morawiecki tweeted on Friday evening that "Stalin was a war criminal, guilty of the death of hundreds of thousands of Poles. Historical truth is not debatable."
"The ambassador of the Russian Federation will be summoned to the Ministry of Foreign Affairs", he said.
On Thursday, Belarus said Wagner mercenaries had started to train Belarusian special forces at a military range just a few miles from the Polish border.
RUSSIA STAGING TACTICAL NUCLEAR WEAPONS
Russia has in recent weeks begun stationing tactical nuclear weapons in Belarus for the first time. The Kremlin said Putin would meet Belarusian President Alexander Lukashenko, with whom he speaks regularly, in Russia on Sunday.
German Defence Minister Boris Pistorius said on Friday that Germany and NATO were prepared to support Poland in defending the military alliance's eastern flank.
Putin said there were press reports of plans for a Polish-Lithuanian unit to be used for operations in western Ukraine - parts of which in the past belonged to Poland - and ultimately to occupy territory there.
"It is well known that they also dream of the Belarusian lands," he said, also without providing any evidence.
On Wednesday, Wagner chief Yevgeny Prigozhin was shown in a video welcoming his fighters to Belarus, telling them they would take no further part for now in the war in Ukraine but ordering them to gather strength for Wagner's operations in Africa while they trained the Belarusian army.
Prigozhin says Wagner, which led the conquest of the Ukrainian city of Bakhmut, is Russia's most effective fighting force. But his frequent clashes with the Moscow defence establishment led him to stage an armed mutiny four weeks ago.
The insurrection ended with an agreement that Wagner fighters - many recruited from prison - could move to Belarus if they wished.
