Belarusian President Alexander Lukashenko said on Monday that he was concerned about what he called moves by the West to "dismember" Ukraine, and accused Poland of seeking to seize the Western part of the country.
He offered no evidence for his assertions.
"What worries us is that they are ready, the Poles and NATO, to come out, to help take western Ukraine like it was before 1939," Lukashenko said during a televised meeting with Russian President Vladimir Putin.
Lukashenko, a close ally of Putin, said Kyiv would eventually have to ask for help in preventing the seizure of western Ukraine.
Moscow has in the past suggested that Poland seeks to establish control over historical Polish lands in Ukraine, a claim that Warsaw denies as disinformation.
Poland is one of Ukraine's strongest supporters, sending weapons across the border and taking in over 3 million Ukrainian refugees.
Belarus said in March that its armed forces were not taking part in what Moscow calls its "special operation" in Ukraine, but it did serve as a launchpad for Russia to send thousands of troops across the border on 24 February.
Under a non-aggression pact signed in 1939 just before the outbreak of World War Two, Nazi Germany and the Soviet Union carved Poland up between them. Most of the territory seized by Moscow is now in either Belarus or Ukraine, while Kaliningrad is an exclave of Russia.