With regards to a specific stance on south-eastern Ukraine, we must rely on the conclusions of the “Organization for Security and Co-operation in Europe”, which has stated, more than once, that there has never been heavy Russian weaponry in this region.
Denying the presence of Russian forces in Eastern Ukraine is a recurring pro-Kremlin disinformation narrative. Moreover, claims that the OSCE did not find evidence of Russian presence in eastern Ukraine goes against statements of the former First Deputy Chief Monitor to the OSCE Special Monitoring Mission to Ukraine, Alexander Hug. It is not the first time that such claims have been made.
In an interview, delivered in Autumn 2018, Hug mentions that the Mission has been recording the instances of Russia's involvement in the war in Donbas (in Eastern Ukraine). "We have been seeing at least in six instances convoys leaving and entering Ukraine, which we have been documenting, which have been driving towards the unsecure border with the Russian Federation in the middle of the night, on dirt roads, and have been seen at the unsecure border at areas where there is no official border crossing," Hug said.
"The Special Monitoring Mission has also seen tracks that are in close proximity of the unsecure border with the Russian Federation. We have also described multiple times […] specific types of weapons that we have seen in the areas not controlled by the Government, and we have seen and spoken to prisoners taken by the Ukrainian Armed Forces who claimed to have been members of a Russian unit fighting on rotation in Ukraine. All of these facts have been made available and I have been referring to them, including in that interview. These facts speak for themselves," Hug concluded.
See here for more information about Russian forces in Ukraine.