Russia suffered heavy losses when Ukrainian forces destroyed the pontoon bridge enemy troops were using to try to cross a river in the east, Ukrainian and British officials said.
It is another sign of Moscow’s struggle to salvage a war gone awry.
Ukrainian authorities, meanwhile, opened the first war crimes trial of the conflict.
The defendant, a captured Russian soldier, is accused of shooting to death a 62-year-old civilian in the early days of the war.
Ukraine’s airborne command released photos and video of what it said was a damaged Russian pontoon bridge over the Siversky Donets River and several destroyed or damaged Russian military vehicles nearby.
Ukrainian news reports said troops thwarted an attempt by Russian forces to cross the river earlier this week, leaving dozens of tanks and other military vehicles damaged or abandoned.
The command said its troops “drowned the Russian occupiers”.
Britain’s Ministry of Defence (MoD) said Russia lost “significant armoured manoeuvre elements” of at least one battalion tactical group as well as equipment used to deploy the makeshift floating bridge.
“Conducting river crossings in a contested environment is a highly risky manoeuvre and speaks to the pressure the Russian commanders are under to make progress in their operations in eastern Ukraine,” the MoD said in its daily intelligence update.
In other developments, a move by Finland and, potentially, Sweden to join Nato was thrown into question when Turkish President Recep Tayyip Erdogan said his country is “not of a favourable opinion” toward the idea.
He accused Sweden and other Scandinavian countries of supporting Kurdish militants and others Turkey considers terrorists.
Mr Erdogan did not say outright that he would block the two countries from joining Nato, but the military alliance makes its decisions by consensus, meaning that each of its 30 member countries has a veto over who can join.
An expansion of Nato would be a blow to Russian President Vladimir Putin, who undertook the war in what he said was a bid to thwart the alliance’s easterly advance.
The invasion of Ukraine has stirred fears in other countries along Russia’s flank that they could be next.
With Ukraine pleading for more arms to fend off the invasion, the European Union’s foreign affairs chief announced plans to give the country an additional 500 million euros (£425 million) to buy heavy weapons.
The battle for the Donbas has turned into a grinding, village-by-village fight with no major breakthroughs on either side.
In the south, Ukrainian officials claimed another success in the Black Sea, saying their forces took out another Russian ship, though there was no confirmation from Russia and no casualties were reported.
The Vsevolod Bobrov logistics ship was badly damaged but not thought to have sunk when it was hit while trying to deliver an anti-aircraft system to Snake Island, said Oleksiy Arestovych, a Ukrainian presidential adviser.
In April, Ukraine sank the Moskva, a guided missile cruiser that was the flagship of Russia’s Black Sea fleet.
In March, it destroyed a landing ship.
Russian Sgt Vadim Shyshimarin, 21, could get life in prison if convicted at his war crimes trial of shooting a Ukrainian man in the head through an open car window in a village in the Sumy region, in the north east, on February 28, four days into the invasion.
The killing is one of thousands of potential war crimes that Ukrainian prosecutors are investigating.
In a small Kyiv courtroom, scores of journalists watched the start of proceedings, which will be closely watched by international observers to make sure the trial is fair.
The defendant, dressed in a blue and grey hoodie and grey sweatpants, sat in a small glass cage during the hearing, which lasted about 15 minutes and will resume on Wednesday.
Shyshimarin was asked a series of questions, including whether he understood his rights and whether he wanted a jury trial.
He declined the latter.
His Ukraine-assigned lawyer, Victor Ovsyanikov, acknowledged the case against the soldier is strong and has not indicated what his defence will be.
Shyshimarin, a member of a tank unit captured by Ukrainian forces, admitted he shot the civilian in a video posted by the Security Service of Ukraine, saying he was ordered to do so.
In Kharkiv, Ukraine’s second-largest city, lessons are being given in a subway station that has become home for many families.
Children joined their teacher Valeriy Leiko around a table to learn about history art, with youngsters’ drawings lining the walls.
“It helps to support them mentally. Because now there is a war, and many lost their homes … some people’s parents are fighting now,” Mr Leiko said.
In part because of the lessons, he said, “they feel that someone loves them”.
The internet connection was a problem for some, she said.
And “it is hard to concentrate when you have to do your homework with explosions by your window”.