Polish Prime Minister Mateusz Morawiecki warned that Wagner fighters in Belarus could pose as migrants and enter the EU.
Wagner's troops could also facilitate illegal migration of migrants from Belarus, the prime minister said.
Morawiecki said that around 100 Wagner soldiers had settled near the city of Grodno, close to the Polish and Lithuanian borders.
The Wagner troops were diverted to Belarus in accordance with an agreement between Belarusian President Alexander Lukashenko and Russian President Vladimir Putin after the failed insurgency attempt.
Lukashenko has denied EU claims that the would-be migrants are crossing borders with EU countries and causing a migrant crisis in Europe.
But Polish Prime Minister claimed on Saturday that more than 100 fighters of the Wagner group had been transported to an area near its border with Lithuania.
The prime minister claimed that mercenaries could cross the border posing as Belarusian border guards or even migrants to help migrants cross into the EU.
"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.
Morawiecki said there had been 16,000 attempts to cross the border between Belarus and Poland so far this year.
According to the EU border agency Frontex, there were 2,312 illegal border crossings from Belarus into the EU between January and June.
Poland and Lithuania have built single fences on their border with Belarus to prevent both illegal migration and Wagner fighters from crossing the border. Now they are considering closing their borders completely.
According to an anonymous allegation, Minsk encourages migrants from the Middle East to travel by plane to Belarus with the promise of easy access to the EU.
Polish border guards then help Belarusian border guards help the migrants cross illegally into Poland.