Ukrainian President Volodymyr Zelensky has suggested that Russia was "preparing something" in Belarus this summer, using military drills as an excuse.
"This summer Russia is preparing something there, under cover of military exercises," Mr Zelensky told a summit in Poland, without giving details or citing evidence.
Russia and Belarus have announced joint military drills for September.
Meanwhile, Russian troops are trying to carve out a buffer zone in Ukraine's northeastern Sumy region, although so far without substantial success, the regional governor has said.
The Sumy region borders on Russia's Kursk region, where Russian forces have lately recaptured territory seized last year in the Ukrainian military's only major cross-border incursion of the war.
Ukraine has said for months that it is concerned about Russia's troops trying to advance into Sumy.
Russia has said its forces have captured some territory there and have driven all Ukrainian troops out of Kursk.
