The Ukrainian military said that four soldiers were killed in shelling it blamed on Moscow-backed separatists in the east of the country on Friday, further undermining a ceasefire brokered last year.
"Unfortunately, from the beginning of 2021 we see an increase in escalation," Ukrainian President Volodymyr Zelensky wrote in a statement in Telegram late on Friday.
"We cannot allow the peace process to be thrown a hundred steps back," he said, adding that in the near future he hoped to discuss the matter with Russia, Germany and France.
Those nations comprise the Normandy format, which is intended to help resolve the conflict and last met in 2019 shortly after Zelensky was elected.
Ukraine this month called on European allies to intervene to halt an uptick in violence between its army and separatist fighters who broke away from Kiev in 2014 when the Kremlin annexed the Crimean peninsula.
The increase in fighting has raised fears over a fresh outbreak after a ceasefire brokered last July between Kiev and the separatists ushered in several months of relative calm.
Marking the anniversary of a 2014 UN resolution supporting Ukraine's "territorial integrity," 45 countries issued a joint statement Friday condemning "Russia's continued destabilization" of the country.
"Russia is a party to the conflict in eastern Ukraine, not a mediator," said the statement signed by countries including the United States and many EU nations.
The Ukrainian army said the four servicemen were killed when separatist fighters opened fire with mortars and grenades near the village of Shumy north of the separatist's de facto capital Donetsk.
As a result of shelling "four soldiers from the Joint Forces were killed and two were injured," the army said in a statement posted on Facebook.
Zelensky said that Armed Forces Commander-in-Chief Ruslan Khomchak immediately flew to the scene.
The skirmish brings the total number of Ukrainian servicemen killed since the beginning of 2021 to 19 as Kiev accuses Moscow and separatists of using banned military hardware.
The fighting has claimed more than 13,000 lives since 2014, according to the United Nations, but the number of new deaths has fallen in recent years.