Poland has sent troops to its eastern border after accusing Russian ally Belarus of violating its airspace.
- Poland has accused Belarus of flying military helicopters over its airspace
- Belarus says Poland is making up the claim to justify a troop build-up
- About 100 Wagner fighters have moved closer to the border inside Belarus, the Polish PM says
The Belarusian military denied violating the area with military helicopters on Tuesday.
It accused Poland of making up the claim to justify growing border troop numbers.
NATO member Poland is one of Ukraine's most fervent backers in its war with Russia.
Poland's defence ministry said it was sending "additional forces and resources, including combat helicopters".
It said it had informed NATO of the border violation and Belarus's charge d'affaires had been summoned to provide an explanation.
The Polish military initially denied any border violation occurred but later said the intrusion took place "at a very low height, hard to intercept by radar".
Writing on Telegram, Belarus's defence ministry claimed that Warsaw had changed its mind about the incident "apparently after consulting its overseas masters".
"This statement was not backed up by data from Poland," it said.
"The Belarusian defence ministry views it in the manner of an 'old wives tale' and notes there were no border violations by Mi-8 and Mi-24 helicopters."
Residents near the Polish border city of Bialowieza, about 200 kilometres east of Warsaw, shared accounts of what they said were border violations on social media.
Wagner fighters 'increasingly dangerous'
Belarus has allowed Russian President Vladimir Putin to use its territory as a launch pad for the Ukraine invasion.
Belarusian President Alexander Lukashenko has not committed his own troops to the war.
The ex-Soviet state has a long history of animosity with Poland, as does Russia.
Last week, Mr Putin accused Poland of harbouring territorial ambitions on Belarus and said it would consider any attack on its neighbour as an attack on itself.
Mr Lukashenko mockingly told Poland on Tuesday it should thank him for keeping Wagner mercenaries in check.
An unspecified number of mercenaries are now stationed in Belarus after an abortive mutiny against the Kremlin last month.
Poland had already started moving more than 1,000 of its own troops closer to the border.
Mr Lukashenko also joked at a meeting with Mr Putin last month some of the fighters were keen to press into Poland and "go on a trip to Warsaw and Rzeszow".
Polish Prime Minister Mateusz Morawiecki said a group of 100 Wagner fighters had moved closer to the Polish border on Saturday, describing the situation as "increasingly dangerous".