Ukrainian President Volodymyr Zelensky said that Russia had gathered 50,000 troops near Ukraine's northern Sumy region but added that Kyiv had taken steps to prevent Moscow from conducting a large-scale offensive there.
The build-up comes as Russia appears to be gearing up for a summer offensive in Ukraine while Kyiv waits for Moscow to present a memorandum laying out its conditions to proceed with ceasefire talks.
Sumy lies across the border from Russia's Kursk region, where Ukraine previously seized and held a pocket of land for months, before being almost fully pushed out last month, although it says it still holds some areas there.