Belarusian President Alexander Lukashenko said on Monday 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.
Belarus has also begun testing the capabilities and equipment of its armed forces, checking not only active units and equipment but also reserve and stored assets, Belarus Ministry of Defense said on Monday.
"A feature of this stage is the verification of weapons, military and special equipment, which are kept in long-term storage in military units of combat, logistics and technical support," the Ministry of Defense said.
"The verification includes the removal of samples of weapons, military and special equipment from storage and their control run during the commission Marches of various lengths. To carry out these activities, the conscription of persons liable for military service is currently not being carried out," the Ministry added.
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 more than 3 million Ukrainian refugees.
Belarus said in March 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 Feb. 24.
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. Kaliningrad, formerly German East Prussia, became an exclave of Russia.