The EU says it is putting on hold work on a key trade agreement with Ukraine, saying the government's arguments have "no grounds in reality". Protesters have taken to the streets in support of the agreement, which was rejected by the government.
US Senator John McCain told thousands of Ukrainian protesters camped on Kiev's main square on Sunday that Ukraine's destiny lay in Europe and that it would make Europe better.
Senators John McCain and Christopher Murphy both addressed the crowd from a stage.
"We are here to tell you that the American people and the United States Congress stands with the people of Ukraine," Murphy told the crowd.
"The way towards change goes through peaceful protest. And in America we stand in awe of the fact that despite the violence on the square you have remained peaceful in support of change," he added.
There is an urgent need for de-escalation in Ukraine as President Yanukovich tries to calm a country bitterly divided over years of history.
Ukraine's political and economic landscape is stuck between old Soviet masters in Russia and the democratising ideals offered by the EU.