Authorities in Warsaw said they would increase Poland's military presence along the border in response and that they had summoned Belarus' top diplomat in Warsaw in protest.
NATO was also informed of the incident, the Defence Ministry said.
Anxieties have grown in Poland and in neighbouring Lithuania after mercenaries from the Russia-linked Wagner group arrived in Belarus following their short-lived rebellion against the Kremlin in June.
Poland and Lithuania belong to both the European Union and NATO, and have worried about their security with Russian ally Belarus and Ukraine on its eastern border.
For two years, large numbers of migrants and refugees from the Middle East and Africa have been attempting to enter Poland and Lithuania, which view the migration as an attempt by Belarus to create instability.
Earlier on Tuesday, Polish residents in an area near the border with Belarus reported seeing helicopters with Belarusian insignia which they said flew overhead.
Some posted photos of the aircraft.
Poland's military initially denied that they had crossed into Polish airspace.
But by evening, the Defence Ministry acknowledged that "there was a violation of Polish airspace by two Belarusian helicopters that were carrying out training near the border".
It added that Belarus had earlier informed Poland of plans to carry out training exercises in the area.
It said they were flying at a low altitude and were not picked up by radar.
In reaction to the incursion, Defence Minister Mariusz Błaszczak ordered an increase in the number of soldiers along the border as well as an increase in other resources, including combat helicopters, the ministry said.
Separately, the Foreign Ministry said that the chargé d'affaires of Belarus was summoned and that it had "issued a firm protest and called on the Belarusian side to immediately and in detail explain the incident".
"The Polish side emphasised that the incident is perceived as another element of the escalation of tension on the Polish-Belarusian border. Poland expects Belarus to refrain from such activities," the ministry said.
The Defence Ministry of Belarus said in a Telegram channel on Tuesday that its military helicopters had not violated the border with Poland.
"Accusations of a violation of the Polish border by Mi-24 and Mi-8 helicopters of the Belarusian Air Force and air defence forces are farfetched and made by the Polish military and political leadership to justify the build-up of forces and means at the Belarusian border," the ministry said.
with Reuters
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