Vice President Joe Biden on Tuesday called Ukrainian President Victor Yanukovych to express “grave concern” over violence in the streets of Kiev, as clashes between police and opposition protesters escalated.

“He called on President Yanukovych to pull back government forces and to exercise maximum restraint,” said the White House in a statement. “The vice president made clear that the United States condemns violence by any side, but that the government bears special responsibility to de-escalate the situation.”

Reports said at least 13 people had died on Tuesday after police charged protesters who have gathered in Kiev's Independence Square for the last two months to protest the government's close ties with Russia.

Opposition groups want Ukraine to forge closer links with the European Union and have criticized Yanukovych for dropping a trade deal with the EU.

According to reports, Ukrainian authorities have threatened to intensify their crackdown on protesters unless they disband.

Biden has spoken to Ukraine’s president numerous times during the crisis and the White House said he again pressed Yanukovych to sit down with opposition groups and seek a peaceful end to the standoff.

“The vice president further underscored the urgency of immediate dialogue with opposition leaders to address protesters’ legitimate grievances and to put forward serious proposals for political reform,” the statement said. “ The United States is committed to supporting efforts to promote a peaceful resolution to the crisis that reflects the will and aspirations of the Ukrainian people.”

White House press secretary Jay Carney said on Tuesday that the U.S. was “appalled by the violence.”

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