Russian President Vladimir Putin warned Poland that any attack on Belarus will be considered an attack on Russia, in a direct threat to the NATO country televised on Friday.

“Aggression against Belarus will mean aggression against the Russian Federation,” Putin told a televised Security Council meeting on Friday, shown by Reuters. “We will respond to it with all means at our disposal,” he said.

Putin appeared to be responding to Warsaw’s decision this week to re-station military units to the east of the country, closer to the Belarusian border, following the Russian ally’s hosting of Wagner mercenary fighters.

Putin said that Poland appears to have interests in retaking eastern territories it lost to former Soviet Union leader Joseph Stalin, including “a good chunk of Ukraine … to take back the historic lands.” He added that “it’s well known that they dream of Belarusian lands as well.”

Polish Prime Minister Mateusz Morawiecki hit back later on Friday, tweeting that “Stalin was a war criminal, guilty of the death of hundreds of thousands of Poles.” He said that the ambassador of the Russian Federation will be summoned to the Ministry of Foreign Affairs.

Morawiecki’s defense minister defended the relocation of troops on Friday, pointing to reports that the Wagner mercenaries were carrying out training exercises with the Belarusian army.

“Training or joint exercises of the Belarusian army and the Wagner group are undoubtedly a provocation,” said Zbigniew Hoffmann, secretary of the government’s National Security Committee, according to a report by Polish state-run news agency PAP.

Belarus has been Russia’s ally throughout Putin’s war on Ukraine. In addition to hosting the Wagner Group following an insurrection on Moscow led by Yevgeny Prigozhin, Belarusian President Alexander Lukashenko has allowed Putin to station tactical nuclear weapons on its territory.

Germany said Berlin and NATO were prepared to support Poland in defending the eastern border, German Defense Minister Boris Pistorius said on Friday, according to Reuters.

 Separately, a drone attack on an ammunition depot in Crimea prompted an evacuation and brief suspension of road traffic on the bridge linking the peninsula to Russia, Reuters reported. Sergei Aksyonov, the Moscow-installed regional governor, said on Saturday that there was an explosion at the depot in Krasnohvardiiske in central Crimea but reported no damage or casualties, according to the report.

The brief halting of traffic on the Crimean Bridge came five days after blasts there killed two people and damaged a section of the roadway — the second major attack on the bridge since the start of the war.