Two Ukrainian soldiers died and six were wounded in a new uptick of fighting between government troops and Russian-backed rebels in the east following high-profile peace talks, the military said Friday.
The latest casualties came despite efforts to revitalise a long-stalled peace process in the simmering war.
"The situation on the frontline has significantly deteriorated over the past 24 hours," military spokesman Oleksandr Motuzyanyk told reporters.
Separatist forces used heavy weapons including tanks, artillery and large-calibre mortars, the Ukrainian military said.
A series of periodic truce deals have helped lower violence levels in two breakaway regions in Ukraine's industrial east but have not fully ended the bloodshed.
On Monday Germany and France voiced cautious optimism that Russia and Ukraine would take steps to revive the peace process as the four foreign ministers held their first meeting since early last year.
During the talks, Russia and Ukraine formally committed to the key tenets of the 2015 Minsk agreement, brokered by Germany and France.
But on Friday international monitors from the Organisation for Security and Co-operation in Europe (OSCE) accused Kiev and rebels of breaching the accord, urging them finally to fulfil their obligations.
"Withdrawing weapons, disengagement and demining ... will bring a ceasefire and security," Alexander Hug, deputy head of the OSCE ceasefire monitoring mission in Ukraine, told journalists.
"Only with a ceasefire, only with security is peace possible."
The mission also said an armed man had fired an assault rifle at its members as they patrolled a rebel-controlled area in the war zone on Thursday.
More than 10,000 people have been killed since the Moscow-backed insurgency broke out in April 2014 following Russia's annexation of Crimea from Ukraine.
Ukraine and its Western allies accuse Russia of funnelling troops and arms across the border.
Moscow has denied the allegations despite overwhelming evidence that it has been involved in the fighting and gives open political support to the rebels.
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