CNN geolocated a video shared to social media on Monday showing a large column of Russian military vehicles near Matveev Kurgan, a settlement in Russia’s Rostov region.
The vehicles are seen facing north-west, in the direction of the Donbas region.
A senior Ukrainian official on Monday also said a Russian offensive in Ukraine's Donbas region "has already started," warning that Russia continued to amass forces in the region.
In remarks on national television, Vadym Denysenko, adviser to the Minister of Internal Affairs, said:
"From my point of view, this big offensive [in the East] has already started. We have to understand it's not going to be the repetition of Feb 24th, when the first airstrikes and explosions started and we said: 'The war has begun.' The big offensive de-facto has already started."
Ukrainian and Western officials have said in recent days they have observed a redeployment of Russian troops to Donbas following major setbacks for Moscow in a push to take Kyiv.
"Yes, there are still no major battles that are being discussed so much in the past few days. But in general we could say the offensive has already started," Denysenko said.