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 Russian foreign minister Sergey Lavrov, Mr Kerry cited what he said was mounting evidence that separatists were continuing to increase the number of buildings under occupation and take journalists and other 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.
He also urged Russia to call for those occupying buildings to disarm and stand down in exchange for amnesty.
Meanwhile 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.
Pro-Russia insurgents who have seized police stations and other public buildings in eastern Ukraine are defying the call to leave, saying they were not party to the agreement by Ukraine, Russia, the United States and the European Union.
Acting president Oleksandr Turchynov said the two bodies found in Slovyansk bore signs of torture. One of the victims was a member of the city council and a member of Mr Turchynov's party, he said.
Terrorists "are beginning to torture and kill Ukrainian patriots. They are impudently rejecting the calls of not only our country but of all the world's society when they demonstratively mock the decisions taken in Geneva," he said.
"These crimes are being done with the full support and connivance of Russia."
The acting government, which took over after President Viktor Yanukovych fled to Russia in February, says Russia is behind the unrest in eastern Ukraine which it fears Moscow could use as a pretext for an invasion. Last month, Russia annexed Crimea weeks after seizing control of the peninsula.
The Defence Ministry said gunfire hit an observation plane over Slovyansk but the aircraft landed safely without injuries.
Standing alongside prime minister Arseniy Yatsenyuk, Mr Biden called on Moscow to encourage pro-Russia separatists in eastern Ukraine to vacate government buildings and checkpoints, accept amnesty and "address their grievances politically".
He said Russia needs to act "without delay", adding: "We will not allow this to become an open-ended process."
Mr Yatsenyuk was harsher in his characterisation of Russia.
"No country should be able to behave like an armed bandit," he said. "Russia should stick to its international commitments and obligations. They should not behave as gangsters in the modern century."
Hundreds of mourners gathered for the Orthodox funeral services of three pro-Russian militiamen killed in a shoot-out on Easter Sunday near the eastern city of Slovyansk, which is now under the control of pro-Russian insurgents.
Many used the services to vent their anger at the interim government in Kiev, which they blamed for the deaths. The pro-Russian forces contend that the three men were killed by members of the Ukrainian ultranationalist group Right Sector, which helped to overthrow the government in February.
Moscow denies it is behind the troubles in eastern Ukraine and that it has failed to live up to the Geneva agreement.
"Before putting forth ultimatums to us, demanding fulfillment of something within two-three days or otherwise be threatened with sanctions, we would urgently call on our American partners to fully recognise responsibility for those whom they brought to power and whom they are trying to shield, closing their eyes to the outrages created by this regime and by the fighters on whom this regime leans," Mr Lavrov said.
Mr Biden has also announced the United States will provide an additional 50 million dollars to help Ukraine's beleaguered government with political and economic reforms.
The money includes 11 million dollars to help conduct the May 25 presidential election, including voter education, administration and oversight. It also will help fund expert teams from US government agencies to help Ukraine reduce its reliance on energy supplies from Russia. Other technical advisers will help fight corruption.
The White House also announced eight million dollars in non-lethal military assistance for the Ukrainian armed forces, including bomb-disposal equipment, communications gear and vehicles.
In the most high-level visit of a US official to Ukraine since the crisis erupted, Mr Biden met Mr Yatsenyuk and Mr Turchynov privately, as well as with democracy activists.
"I know the men and women who hide behind masks and unmarked uniforms, they do not speak for you," Mr Biden said. "All are welcome as equals in shaping a new Ukraine. We count on you to be the voice for rights and freedoms."