Ukraine’s military says its forces on the eastern frontline have come under heavy rebel shelling, with one soldier killed and four wounded.
But Russian-backed rebels say troops tried to break through their lines and got caught in a minefield, with two killed and three wounded.
Ukraine’s President Volodymyr Zelensky is convening a meeting of his Security Council to assess the flare-up.
The violence puts new pressure on a shaky ceasefire deal.
The clash, now said to be over, took place near Zolote, a frontline village in the Luhansk region of Donbas. The conflict began in 2014.
The rebels hold much of Donbas.
A framework ceasefire deal was agreed in February 2015 – the Minsk agreement – but was soon violated as fighting resumed.
The Ukrainian Joint Forces Operation HQ, commanding government troops in Donbas, says its positions were shelled heavily at several points around Zolote on Tuesday: Novotoshkivske, Orikhove, Krymske and Khutir Vilny.
“The occupiers are firing on our defenders using weapons banned under the Minsk [ceasefire] agreements. The enemy is using 120mm mortars and firing various grenade launchers and heavy machine-guns. Under the cover of firing, the Russian occupiers have switched to offensive actions and have tried to cross the contact line,” the JFO statement said.
A local official, Vitaliy Shvedov, said the intense shelling was similar to the peak of fighting in 2015.
The JFO said its forces had not lost ground to the rebels who, it said, had requested a ceasefire.
In recent months there has been cautious progress towards a peace settlement: there were significant prisoner exchanges and talks between Mr Zelensky and Russian President Vladimir Putin, mediated by France and Germany.
Mr Zelensky called Tuesday’s flare-up “an attempt to wreck the peace process in Donbas, which had started to move forward with small but continuous steps”.
More than 13,000 people have died in the conflict, which dates back to April 2014, when the separatist rebels seized key areas of the Donetsk and Luhansk regions.
The rebels have Russian heavy weapons and Western experts say Russian regular units have bolstered their ranks. The Kremlin denies that, while admitting that Russian “volunteers” are helping the rebels.