BRUSSELS — The European Union on Friday blamed Russia for the deaths of at least five troops in eastern Ukraine in what appears to be the worst outburst of violence in months.

“The death of five Ukrainian servicemen yesterday is the just the latest proof of the tragic consequences of Russia’s aggression in Ukraine,” said EU Council President Donald Tusk after a summit with six eastern European nations, including Ukraine.

Fighting between Russia-backed separatists and Ukrainian government troops has killed more than 10,000 people since it began in 2014. A cease-fire deal signed in 2015 helped to reduce the scale of fighting but did not stop it altogether. In the meantime, Russia has annexed the Ukraine peninsula of Crimea.

“Frozen and armed conflicts continue to prevent development and create hardships in eastern partnership countries. The EU condemns Russia’s aggression and will never recognize the illegal annexation of Crimea,” Tusk said.

The EU has already imposed restrictive measures against Russia.

The press office of the Ukrainian armed forces said in a statement Friday four out of the five troops killed in the past 24 hours died outside the village of Krymske in the Luhansk region where the fighting lasted for eight hours. The Ukrainian government called it the biggest loss of life since July.

The Organization for Cooperation and Security in Europe, which monitors the shaky truce in eastern Ukraine, said on Friday there was a spike in violence in eastern Ukraine.

Alexander Hug, first deputy chief of the OSCE’s monitoring mission in eastern Ukraine, said in a statement that the European monitors last week recorded the highest weekly number of cease-fire violations since mid-June including 360 instances of the use of heavy weapons that should have been withdrawn

“With 400+ civilians killed and injured this year because of the fighting in eastern Ukraine, the sides need to ensure the well-being of the civilian population,” Hug said.