Ukraine said Friday five of its soldiers had died during fighting in the east, as it accused Russia of ramping up its military presence in the region amid squabbles among warring rebel factions.
The spike in fighting came as leaders from the European Union and six former Soviet countries were meeting in Brussels for a summit aimed at deepening ties, with Moscow's role in the Ukraine conflict conspicuously off the agenda.
But Ukrainian President Petro Poroshenko discussed the simmering conflict with German Chancellor Angela Merkel during his visit to Brussels, his office said.
The two leaders touched upon the "dangerous development of the situation in Donbass (in eastern Ukraine) which was provoked by ceasefire violations by Russian militants," Poroshenko's office said in a statement.
Secretary of Ukraine's Security and Defence Council Oleksandr Turchynov claimed that Moscow had reinforced its presence in eastern Ukraine as infighting has increased between warring rebel factions loyal to the Kremlin.
It was the highest daily death toll since August 25 when a new truce entered into force in the latest effort to put an end to a conflict that has claimed more than 10,000 lives since 2014.
"Five of our soldiers have died over the past 24 hours," Turchynov said in televised remarks. "It is a very serious problem for us."
"A conflict situation is under way between various criminal groups which make up the occupational administration in the occupied territories," he added.
Separately, the Ukrainian military's anti-terror centre said that four of the soldiers died in the Lugansk region, one of two regions controlled by Moscow-backed rebels, and that four more were wounded.
It said eight rebels had been killed and another nine wounded.
The Kiev anti-terror centre said the army remained in control but the situation was "tense" and a ceasefire was being violated.
Tensions broke out earlier this week in the self-proclaimed Lugansk People's Republic, which has for years been blighted by leadership squabbles.
The region's separatist leader, Igor Plotnitsky, has sacked his interior minister and accused him of masterminding a coup attempt.
The minister, Igor Kornet, refused to step down and armed men loyal to him blockaded part of the Lugansk city centre for two days earlier this week before dissipating on Thursday.
The current whereabouts of Plotnitsky were not known Friday and his official website was down.
Russian nationalist writer Zakhar Prilepin, who has joined the rebels, wrote on Facebook the day before that he had seen him flying to Moscow, and videos allegedly showing Plotnitsky in the airport were shared on social media.
Kremlin spokesman Dmitry Peskov refused Friday to comment.
"Plotnitsky's arrival in Moscow is not an issue that the presidential administration concerns itself with."
He was quick however to stress that Moscow was committed to the moribund EU-brokered Minsk agreement to end the Ukraine conflict.
Alexander Hug, deputy director of the OSCE monitoring mission in Ukraine, said on Friday that the level of violence in eastern Ukraine had increased.
He said that last week monitors "recorded the highest weekly number of ceasefire violations since mid-June - more than 9,000 ceasefire violations, which is 60 percent more than the previous week."
Hug also noted that tensions had decreased in Lugansk and pointed to "reduced armed presence compared to earlier in the week."t