Poland is sending thousands of extra troops to its border with Belarus following an alleged incursion by Belarusian military helicopters.
Poland's government says fighters from the Wagner mercenary group, who are in Belarus, have been moving towards the border.
What was the latest incident on the Poland-Belarus border?
Poland's government says that on 1 August, two Belarus military helicopters flew at low altitude about two kilometres into its territory in the Bialowieza region.
Belarusian armed forces were conducting exercises in the border area at the time.
Its government said that there were "no border violations by Mi-8 and Mi-24 helicopters", and described the accusation as an "old wives' tale".
Residents of Bielowieza have posted photographs on social media of a Mi-8 helicopter and a Mi-24 helicopter with Belarusian markings, which they say flew over the town.
BBC Verify has matched the serial numbers on the helicopters to ones seen at the nearby Machulishchi airfield in 2018.
This military incursion comes on top of thousands of illegal border crossings into Poland from Belarus by migrants.
Poland claims that since 2021 Belarus has been encouraging people from the Middle East and Africa to travel to Belarus and cross the border illegally.
Even though there are fewer crossings now than two years ago, Poland's border guard says 19,000 migrants have tried to cross the border so far this year, and 16,000 did so last year.
Poland's government has called Belarus' strategy "hybrid warfare".
Could Russian Wagner mercenaries be preparing to enter Poland?
Following the failed mutiny last June by mercenaries from Russia's Wagner group, a number of its troops have moved into Belarus.
The leader of Belarus, Alexander Lukashenko, joked in a conversation with Russia's President Vladimir Putin: "They are asking to go west... to go on a trip to Warsaw... But of course, I am keeping them in central Belarus, like we agreed."
Polish Prime Minister Mateusz Morawiecki said a group of 100 Wagner fighters had been moving towards Grodno, a city in the north-west of Belarus on the Polish border, and described the situation as "increasingly dangerous".
He warned that Wagner fighters in Belarus could enter Poland posing as migrants, or that they could pose as Belarusian border guards and help more illegal migrants to cross.
Belarus' defence ministry said that Wagner troops were present in the Brestsky camp in the south of the country, about 10km from the Polish border, and were training Belarusian troops.
"Being mercenaries, its troops could create disturbances on the border which Russia and Belarus could not be directly blamed for," says Dr Barbara Yoxon, a lecturer in international relations at the University of Lancaster.
Why is this border region so sensitive?
The border between Poland and Lithuania is a line called the Suwalki Gap.
This 60-mile (95km) strip of land is also the link between Belarus and the heavily-fortified Russian exclave of Kaliningrad.
Many military analysts see the Suwalki gap as a potential flashpoint area if there were to be a conflict between Nato countries and Russia.
The fear is that if Russia, along with Belarus, were to use military force to close this gap, they could cut off the Baltic republics - Lithuania, Latvia and Estonia - from their Nato allies in Europe.
"This is a very narrow choke-point," says Dr Yoxon. "Russia and Belarus together would find it easy to block Nato from sending reinforcements to protect the Baltic states."
Few military analysts believe that these alleged incursions indicate that Russia is aiming to attack the Baltic republics.
However, says Professor Malcolm Chalmers from the think tank, the Royal United Services Organisation: "It's partly an exercise by Russia and Belarus to test the water and see how Nato might respond, or not respond, to incursions into Nato territory."
Russia and Belarus have signed a number of military accords in recent years. Belarus allowed Russian troops to invade Ukraine from across its border, and is allowing Russia to put tactical nuclear missiles on its soil.
Dr Anais Marin from the international affairs think tank, Chatham House, says the alleged incursions were probably a Russian idea, "to make sure that Belarus remains an adversary of Poland and Nato, and a firm partner of Moscow's."
What is Poland doing in response to the border tensions?
Poland's defence ministry says it is sending 10,000 additional troops to the Belarus border. It says 4,000 will directly support the Border Guard, and 6,000 will be held in reserve.
In July, the government sent an extra 1,000 troops to the border to guard against potential incursions from Wagner fighters.
However, some political analysts suggest that Poland's politicians may be exaggerating the threats from Wagner and Belarus because they are facing elections shortly, and want to show that they are tough on security issues.