President Vladimir Putin on Friday accused NATO member Poland of having territorial ambitions in the former Soviet Union.

He 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.

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 Africa while they trained the Belarusian army.

Prigozhin says Wagner, which led the grinding 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, would either move to Belarus, sign up with the Russian army or return to their homes.

On Thursday, Belarus said Wagner mercenaries had started to train Belarusian special forces at a military range just a few miles from the border with Poland.

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.

"But as far as Belarus is concerned, it is part of the Union State (with Russia); unleashing aggression against Belarus will mean aggression against the Russian Federation," Putin said. "We will respond to this with all the means at our disposal."