A failed river crossing where a Russian battalion lost almost all its armoured vehicles was the third such attack in three days, a Ukrainian army official has told the BBC.
Serhiy Hayday, head of the Luhansk Regional Military Administration, said Russia was trying hard to cross the Siversky Donets in order to encircle a strategic east Ukrainian city.
But he claimed that in days of intense battles, local forces had destroyed speed boats and helicopters and "ruined Russian boat bridges three times".
Images from the scene show dozens of burnt-out armoured vehicles on the banks of the river.
Ukrainian information service, InformNapalm, said that Russia began its crossing attempt on 8 May.
But as soon as they crossed, Ukrainian forces destroyed the pontoon bridge in the water, and Russian troops "fell into the trap", it said.
The UK ministry of defence claimed on Friday that the incident - which it said was a "highly risky" attempt to cross the water - revealed the pressure Russian commanders were under to make progress.
The river flows through the Donestsk and Luhansk regions - both major Russian military targets.
Moscow has not yet commented on the attacks, and the BBC has not independently verified Mr Hayday's account of the battles.
But the official described "a great accumulation" of Russian troops making repeated attempts to encircle Ukrainian forces.
"We still can't bring down Russia's offensive pace," he said. "Russia continues to attack in this direction, and we are forced to defend ourselves."
In repeated attacks, Ukrainian troops had "eliminated approximately 70 units of Russian heavy weapons and equipment", disrupting attempts to cross.
It isn't clear how many Russian soldiers were killed in the battalion, which would typically consist of more than 500 troops.
Crossing the river, is a vital move to surround Ukrainian troops and take important cities such as Severodonetsk, according to analysts.
Oleh Zhdanov, a Ukrainian military expert told the BBC: "If they take this crossing, cross the river in this area, it allows the Russian Federation to further advance to the estuary. The advance towards Severodonetsk is the plan they have come up with."
Severodonetsk is important to the Russians because controlling it would enable Russia to drive west and link up with Russian forces pushing south-east of Izyum.
Despite the destruction on the banks of the Siversky Donets, Russian troops are making progress in nearby areas.
Mr Hayday said Moscow's forces had taken the town of Rubizhne and that the national guard was attempting to defend the outskirts of the town.
"Russians are doing their best to eliminate our fighters, but they are not able to do this," he said. "Ukrainian positions are still very strong there," he said.
Meanwhile, in another area of eastern Ukraine, Ukrainian forces have driven back Russian troops from the second-largest city of Kharkiv.
It is thought to be the fastest advance since Russian forces pulled away from Kyiv more than a month ago.