The violence came a day after interim Ukrainian President Oleksandr Turchynov announced that Kiev would resume "counter-terrorism" efforts in the eastern region of the country. Those efforts have been backed by the United States.
The Ukrainian military suspended its crackdown against the pro-Russian separatists last week, after it signed an agreement with Russia, the United States and the European Union offering amnesty to any militants who laid down their arms.
But militants have not disassembled checkpoints or left occupied buildings, prompting the U.S. and Ukraine to complain that Moscow had reneged on the agreement.
At a press conference earlier Thursday in Japan, President Obama said Russia had failed to "abide by the spirit or the letter of the agreement in Geneva."
"Instead, we continue to see militias and armed men taking over buildings, harassing folks who are disagreeing with them, and destabilizing the region, and we haven’t seen Russia step up and discourage that," Obama said.
The president warned that the U.S. had “teed up” additional sanctions against Russia, and said Moscow had “days, not weeks” to publicly renounce the actions of “malicious and armed” pro-Russian separatists who continue to hold government buildings in eastern Ukraine.
Russian President Vladimir Putin, in turn, denounced the military action undertaken by Kiev.
“If the Kiev regime has started to use the army against the population inside the country, it, beyond any doubt, is a very serious crime,” Putin said at a media forum in St. Petersburg, according to The Washington Post.
Putin also warned the Ukrainian action would have "consequences" and said that the effort was "proof that we have acted correctly and on time" in taking over Crimea. Moscow's annexation of the ethnically Russian peninsula has been widely denounced as a violation of international law by international powers, including the United States.
Russia's defense minister also announced a new set of military exercises along the Ukrainian border following the clashes in Slovyansk.
"We have to react to these developments somehow," Russian Defense Minister Sergei Shoigu said according to The Associated Press.