Secretary John Kerry has warned Russian Foreign Minister Sergei Lavrov that Washington will impose more sanctions on Russia if tensions did not de-escalate in eastern Ukraine, a US official told the Reuters news agency.
The telephone call between the two follows a vow from Ukraine's interim government to step up anti-terror operations after the body of a pro-unity politician was allegedly found brutally tortured.
Tensions remain high in eastern Ukraine after at least three died in a day of shooting during what was supposed to be an Easter truce.
In an exclusive interview with NBC News, the Ukrainian Prime Minister warns of the dangers of unchecked Russian expansion and aggression.
Russia, the US and the EU say that all sides have agreed to steps to "de-escalate" the crisis in Ukraine at talks in Geneva.