A deal designed to end Ukraine’s long-running crisis was signed Friday afternoon by President Viktor Yanukovych and the three leaders of the political opposition, but the accord appeared likely to be a hard sell among the thousands of demonstrators who have sustained months of protest.

Violent clashes between hard-line protesters and police erupted Tuesday in Kiev, the Ukrainian capital, after more than a week of relative calm, leaving at least 13 people dead and many more wounded.

Violent clashes between hard-line protesters and police erupted Tuesday in Kiev, the Ukrainian capital, after more than a week of relative calm, leaving at least 13 people dead and many more wounded.

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.

Washington announced no specific new action, but U.S. Ambassador to Ukraine Geoffrey Pyatt threatened both sides with sanctions.

“We believe Ukraine’s crisis can still be solved via dialogue, but those on both sides who fuel violence will open themselves to sanctions,” Pyatt said on Twitter, in both English and Russian.

Carney urged Yanukovych to resume a political discussion with opposition leaders as police and protesters clashed in the worst day of violence since opposition supporters set up camp in Kiev’s Independence Square in November.

“We continue to condemn street violence and excessive use of force by either side. Force will not resolve the crisis,” Carney said.

Pyatt retweeted an appeal to Yanukovych by Swedish Foreign Minister and former Balkan peace negotiator Carl Bildt.

“Only person who can now stop catastrophe in Ukraine is President Yanukovych,” Bildt said on Twitter. “His vacillation and violence responsible for situation.”

Pyatt told CNN that the United States is not “meddling.” “We are Ukraine’s friend,” he said.

The main anti-government protest camp in Kiev was on fire Tuesday after Ukrainian riot police started to move into Independence Square late Tuesday. Six police officers were among the dead after a chaotic day of street battles. Hundreds were injured in street clashes.

Television footage showed riot police throwing stun grenades at protesters across a line of burning tents, tires and debris. Riot police used water cannons to try to clear the square, and police shut down subway stations and blocked traffic. Protesters shouting “Glory to Ukraine” set fires and threw small homemade explosives.

The violence followed Russia’s announcement Monday that it would to resume loans to the Yanukovych government. Opponents have been demanding Yanukovych’s resignation because of his choice late last year to withdraw from a planned economic union with Europe and instead accept $15 billion in loans from Moscow.