Fierce fighting in eastern Ukraine on Tuesday killed one Ukrainian soldier and wounded four, Kiev said, in some of the worst clashes in years with Russian-backed separatists.
Ukrainian President Volodymyr Zelensky said the bloodshed was a "cynical provocation" that undermined new efforts to resolve the conflict.
The soldiers had come under fire from weapons banned under a ceasefire agreement, the military said.
It said separatists used artillery, mortars and grenade launchers against Ukrainian positions near the villages of Novotoshkivka and Krymske, less than 10 kilometres from one of three zones from which both sides agreed to withdraw last year.
"This is one of the biggest escalations in the last few years," Ukrainian military spokesman Dmytro Chalyi told AFP.
During a press briefing after the clashes, Zelensky praised the Ukranian military for its "strong and adequate response", and said a lasting ceasefire would be brokered soon.
Kremlin spokesman Dmitry Peskov said both sides had suffered casualties, adding that Moscow "does not know the details of what provoked the clash."