The United States condemned an explosion of street violence in Ukraine that killed at least 15 people Tuesday and said the government bears primary responsibility for restoring calm.

Vice President Biden called Ukrainian President Viktor Yanukovich to express what the White House termed “grave concern,” and called on the embattled leader to pull back government forces after a day of chaotic street clashes and immediately resume political discussions with opponents.

Biden “made clear that the United States condemns violence by any side, but that the government bears special responsibility to de-escalate the situation,” a White House statement said.

White House press secretary Jay Carney said the Obama administration is “appalled” by the violent crackdown on anti-government protesters in the Ukrainian capital.