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."
The leader of the self-proclaimed Lugansk People's Republic Leonid Pasechnik said Kiev was responsible for "bloody provocations" and "aggressive behaviour".
The US embassy in Kiev expressed "deep concern" over the escalation and called on Russia to uphold its commitments under recent ceasefire agreements.
Ukraine has been fighting separatists in the eastern Donetsk and Lugansk regions since 2014 following Moscow's annexation of the Crimean peninsula. The insurgents have received support from Russia, which was slapped with heavy sanctions by the West over its role in the conflict.
More than 13,000 people have been killed since fighting broke out in 2014, despite peace accords signed in the Belarus capital Minsk in 2015.
Zelensky, a political newcomer, was elected president last year after vowing to end the fighting.
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