More evidence of the involvement of Russian regular armed forces comes from a satellite imagery research effort.
The latest study by a prominent OSINT team shows proof Russia has been flying its military helicopters in and out of the occupied areas of eastern Ukraine.
Experts from Glasnost Gone have put together a short video showing "good evidence" Russia has used military helicopters in Ukraine.
Read alsoUkraine intel says militants cross to Russia to get more armored vehicles
"One advantage for the use of helicopters, would be the rapid transportation of wounded soldiers, Special Forces and Russian military commanders," the report says.