Wagner fighters in Belarus could pose as migrants and enter the EU, Polish Prime Minister Mateusz Morawiecki has warned.
Wagner could also facilitate illegal migration from Belarus, which Poland describes as "hybrid warfare", he says.
About 100 Wagner troops have moved near the city of Grodno, close to the Polish and Lithuanian borders, the PM added.
Some Wagner troops have moved to Belarus under a deal to end a brief mutiny in Russia in June.
Warsaw says it sees Wagner's presence in Belarus as a potential threat and is seeking to shore up its eastern flank.
Belarusian leader Alexander Lukashenko has previously denied provoking a migrant crisis in Europe by luring would-be migrants to its borders with EU nations.
But Mr Morawiecki said on Saturday that more than 100 members of the Wagner group had moved to north-western Belarus near the Suwalki gap - Poland's 60-mile (95km) border with fellow EU state Lithuania, which separates Belarus and the Russian exclave of Kaliningrad.
He claimed the mercenaries might pose as Belarusian border guards in order to help migrants cross into the EU, or even pretend to be migrants themselves to enter the bloc.
"Now the situation is becoming even more dangerous," he told a news conference on a visit to an arms factory in Gliwice, southern Poland.
"This is certainly a step towards a further hybrid attack on Polish territory," he added.
Several thousand Wagner fighters have moved to Belarus since the group's short-lived mutiny against the Kremlin in June. They were offered a choice of joining the regular Russian army or heading to Belarus, a close ally of Russia.
On Thursday Poland's interior minister said Poland, Lithuania and Latvia could jointly decide to shut their borders with Belarus if there were incidents involving the Wagner group along their frontiers.
Last weekend Mr Lukashenko insisted he would keep the Wagner mercenaries in central Belarus.
"They are asking to go west... to go on a trip to Warsaw... " Mr Lukashenko joked in a conversation with Russian President Vladimir Putin.
"But of course, I am keeping them in central Belarus, like we agreed."___