US secretary of State John Kerry has told Russia he is deeply concerned over what he describes as the lack of positive steps by Moscow to de-escalate the violence in eastern Ukraine.
In a phone call to Russian foreign minister Sergey Lavrov, Mr Kerry cited what he said was mounting evidence separatists were continuing to increase the number of buildings under occupation and take civilians captive.
The US State Department said Mr Kerry urged Russia to tone down its rhetoric and engage diplomatically in the east with the Ukrainian government and the Organisation for Security and Co-operation in Europe.
Ukraine’s acting president ordered security forces to resume operations in the east after the bodies of two people allegedly abducted by pro-Russia insurgents were found and a military aircraft was reportedly hit by gunfire.
The developments - just hours after US vice president Joe Biden left the Ukrainian capital Kiev - raised fears that last week’s international agreement on easing the crisis was unravelling.
The accord calls for all sides to refrain from violence and for demonstrators to vacate public buildings. It does not specifically ban security operations, but Ukraine suspended its so-called “anti-terrorist operation” after it was reached.