Pro-Russian separatists fired rockets at a Ukrainian military camp in eastern Ukraine before dawn Friday, killing at least 19 troops, a Ukrainian military spokesman said.
The attack happened around 4:30 a.m. near the town of Zelenopolsk, in eastern Ukraine's restive Luhansk region, military spokesman Yuriy Stets said.
At least 55 other people were injured, according to the nation's counterterrorism office.
The office of President Petro Poroshenko, whose country has battled separatists in the east for months, accused attackers of using Russian-made Grad rockets against Ukrainian armed forces.
"Militants will pay hundreds of their lives for each life of our servicemen," Poroshenko said. "Not a single terrorist will avoid responsibility; each of them will be punished."
The European Union is sanctioning 11 more people over the Ukraine crisis, the European Council said Friday, bringing the total number subject to EU visa bans and asset freezes because of the crisis to 72.
The new names will be announced Saturday morning, the council said.
Those already targeted include people in Ukraine and Russia, including Vyacheslav Ponomaryov, the self-declared mayor of Slovyansk, a rebel stronghold in the Donetsk region, and Vladimir Shamanov, commander of Russian airborne troops.