Vice President Biden on Tuesday called Ukranian leader Viktor Yanukovych to express "grave concern" about a brutal police crackdown in Kiev.
According to the White House, Biden urged Yanukovych to "pull back government forces and to exercise maximum restraint."
Biden and Yanukovych have remained in regular contact throughout the political crisis that has gripped the streets of Ukraine's capital city.
The State Department issued a travel alert Tuesday warning U.S. citizens of "the increased risks of travel to Ukraine because of the ongoing political unrest and violent clashes between police and protesters," the department said in a statement.
"U.S. citizens are urged to maintain a low profile and to remain indoors at night while clashes continue," the statement read.
On Tuesday, at least 13 people — including six policemen — were killed in crashes between government forces and protesters, according to the BBC.
The unrest began earlier this year when Yanukovych opted against signing a trade deal with the European Union in favor of a bailout financed by Russia. Opposition demonstrators have called for his resignation and new elections.
Earlier Tuesday, White House press secretary Jay Carney said he was "appalled by the violence" taking place in downtown Kiev.
"We continue to condemn the street violence and excessive use of force by either side. The force will not resolve the crisis," Carney said.