"There was no first line of defense," Denys Yaroslavskyi told the BBC, which reported from Vovchansk on Sunday.
Jonathan Beale, a defense correspondent at the outlet, wrote that Yaroslavskyi showed him drone footage of Russian troops walking past Ukraine's northeastern border without resistance.
Lying on Kharkiv's northeastern border, Vovchansk is one of Ukraine's closest cities to the Russian region of Belgorod.
Related storiesDays earlier, Ukrainian military observers reported that between 30,000 to 35,000 Russian troops had gathered for the push.
The Kremlin has since 2022 been accusing Ukraine of shelling Belgorod, though this also comes amid repeated reports of Russian troops misfiring or dropping bombs by mistake on civilians there.
Persons:
—, Denys Yaroslavskyi, Jonathan Beale, Yaroslavskyi, Oleksandr Syrskyi, Syrskyi, Vladimir Putin, Vovchansk
Organizations:
Service, BBC, Business, Reconnaissance Unit, Armed Forces, Ukraine's General Staff, Russian Federation, Kyiv, Russian, NATO
Locations:
Ukrainian, Kharkiv, Russia, Vovchansk, Ukraine's, Russian, Belgorod, Moscow, Ukraine, The Washington, Kyiv