"There was a violation of Polish airspace by two Belarusian helicopters that were training near the border," the Polish defence ministry said in a statement.

Earlier on Tuesday, Poland's army said the helicopters had not violated the NATO country's airspace, but it later backtracked in a subsequent statement.

"The violation took place in the Bialowieza area at a very low altitude, hampering the detection by radar systems," the ministry said in explanation.

It added that "NATO was notified about the incident".

It also said the defence minister ordered an increase of the number of soldiers at the Belarusian border, citing intensified "hybrid actions" by Russia and Belarus against Poland.

Poland and its neighbours on NATO's eastern flank have voiced concerns about a potential threat from Belarus which now hosts the Wagner mercenary group.

On Saturday, Polish Prime Minister Mateusz Morawiecki said a group of "one hundred Wagner mercenaries" in Belarus had moved closer to the Polish border.

Morawiecki called it "a step towards further hybrid attack on the Polish territory".

"They will probably be disguised as Belarusian border guards, helping illegal migrants to cross into Polish territory, to destabilise Poland," Morawiecki said.