Around 20 Ukrainian soldiers were killed on Friday when their convoy was ambushed by pro-Russia separatist rebels.
Another 13 were injured and the fate of 11 other paratroopers is unknown.
The attack happened in Shakhtars, close to the Malaysia Airlines crash site.
National Security and Defence Council spokesman Andriy Lysenko said: “They are trying to keep Shakhtarsk and won’t let the Ukrainian military get there. Because Shakhtarsk is a key point which divides Donetsk region from Luhansk region.”
Meanwhile the Ukrainian Defence Ministry released a video of what it says are confiscated rebel weapons, ammunition, and vehicles from eastern Ukraine.
Kyiv has intensified its military offensive against the rebels since the Malaysia Airlines plane was shot down.
The Ukrainian government said more than 100 international experts from the Netherlands, Australia and OSCE were working at the crash site on Friday.