The Russian movements were geolocated and the social media videos authenticated by CNN. According to the broadcaster, some of these videos are from official sources but most from TikTok or YouTube.

There's plenty of other evidence of the military build-up in the Belgorod region too. CNN geolocated these videos at the village of Sereteno, about 15 miles (24 kilometers) from the border.

They were uploaded on Sunday and show tanks moving through the area. Russian forces are also moving at night in the area -- with tanks being offloaded from trains, CNN reported.

According to videos, large collection of tanks and infantry fighting vehicles were also seen. CNN quoted analysts as saying that the tanks were part of the Russian 1st Guards Tank Army.

Michael Kofman, Russia analyst at CNA, a nonprofit organization based in Virginia, said that the units in Kursk (such as the 6th Combined Arms Army) "appear to be moving up to Belgorod with district level artillery support.

The 1st Guards Tank Army has started to move "presumably to a final staging area", he added. The Russian forces have also taken up positions close to where the borders of Ukraine, Russia and Belarus meet, CNN reported.